Remove unused `toRange` implementation

This has been replaced by the `TextRange.toRange` method in `text-range.js`.
The test cases are covered by the tests for `TextRange` and
`TextPosition`.

src/annotator/anchoring/test/text-position-test.js

-import { toRange } from '../text-position';
-
-describe('text-position', () => {
-  let container;
-
-  before(() => {
-    container = document.createElement('div');
-    container.innerHTML = `<h1>Test article</h1>
-<p>First paragraph.</p>
-<p>Second paragraph.</p>`;
-  });
-
-  after(() => {
-    container.remove();
-  });
-
-  describe('toRange', () => {
-    const testCase = (description, text) => ({
-      description,
-      text,
-      expected: text,
-    });
-
-    [
-      testCase('start text of root', 'Test article'),
-      testCase('a whole text node', 'First paragraph.'),
-      testCase('end text of root', 'Second paragraph.'),
-      testCase('part of a text node', 'rst paragraph'),
-      {
-        description: 'negative start offset',
-        start: -5,
-        end: 5,
-        expected: new Error('invalid start offset'),
-      },
-      {
-        description: 'invalid start offset',
-        start: 1000,
-        end: 1010,
-        expected: new Error('invalid start offset'),
-      },
-      {
-        description: 'invalid end offset',
-        start: 0,
-        end: 1000,
-        expected: new Error('invalid end offset'),
-      },
-      {
-        description: 'an empty range',
-        start: 0,
-        end: 0,
-        expected: '',
-      },
-      {
-        description: 'a range with end < start',
-        start: 10,
-        end: 5,
-        expected: '',
-      },
-    ].forEach(({ description, start, end, expected, text }) => {
-      it(`returns a range with the correct text (${description})`, () => {
-        if (text) {
-          start = container.textContent.indexOf(text);
-          end = start + text.length;
-        }
-
-        if (expected instanceof Error) {
-          assert.throws(() => {
-            toRange(container, start, end);
-          }, expected.message);
-        } else {
-          const range = toRange(container, start, end);
-          assert.equal(range.toString(), expected);
-        }
-      });
-    });
-  });
-});

src/annotator/anchoring/text-position.js

-/**
- * Functions to convert between DOM ranges and characters offsets within the
- * `textContent` of HTML elements.
- *
- * These were added to work around issues in `dom-anchor-text-position`'s
- * `toRange` implementation. When the issue is resolved upstream, we may still
- * want to keep the test suite for this module.
- *
- * See https://github.com/hypothesis/client/issues/1329
- */
-
-/**
- * Convert `start` and `end` character offset positions within the `textContent`
- * of a `root` element into a `Range`.
- *
- * Throws if the `start` or `end` offsets are outside of the range `[0,
- * root.textContent.length]`.
- *
- * @param {HTMLElement} root
- * @param {number} start - Character offset within `root.textContent`
- * @param {number} end - Character offset within `root.textContent`
- * @return {Range} Range spanning text from `start` to `end`
- */
-export function toRange(root, start, end) {
-  // The `filter` and `expandEntityReferences` arguments are mandatory in IE
-  // although optional according to the spec.
-  const nodeIter = root.ownerDocument.createNodeIterator(
-    root,
-    NodeFilter.SHOW_TEXT
-  );
-
-  let startContainer;
-  let startOffset = 0;
-  let endContainer;
-  let endOffset = 0;
-
-  let textLength = 0;
-
-  let node;
-  while ((node = nodeIter.nextNode()) && (!startContainer || !endContainer)) {
-    const nodeText = /** @type {string} */ (node.nodeValue);
-
-    if (
-      !startContainer &&
-      start >= textLength &&
-      start <= textLength + nodeText.length
-    ) {
-      startContainer = node;
-      startOffset = start - textLength;
-    }
-
-    if (
-      !endContainer &&
-      end >= textLength &&
-      end <= textLength + nodeText.length
-    ) {
-      endContainer = node;
-      endOffset = end - textLength;
-    }
-
-    textLength += nodeText.length;
-  }
-
-  if (!startContainer) {
-    throw new Error('invalid start offset');
-  }
-  if (!endContainer) {
-    throw new Error('invalid end offset');
-  }
-
-  const range = root.ownerDocument.createRange();
-  range.setStart(startContainer, startOffset);
-  range.setEnd(endContainer, endOffset);
-
-  return range;
-}