Remove unused code in src/sidebar/filter/

This used to contain Angular filters. The last references to this were
removed in c978b6b2.

src/sidebar/filter/test/url-test.js

-import * as url from '../url';
-
-describe('url.encode', function() {
-  it('urlencodes its input', function() {
-    const expect = 'http%3A%2F%2Ffoo.com%2Fhello%20there.pdf';
-    const result = url.encode('http://foo.com/hello there.pdf');
-
-    assert.equal(result, expect);
-  });
-
-  it('returns the empty string for null values', function() {
-    assert.equal(url.encode(null), '');
-    assert.equal(url.encode(undefined), '');
-  });
-});

src/sidebar/filter/url.js

-/**
- * URL encode a string, dealing appropriately with null values.
- */
-export function encode(str) {
-  if (str) {
-    return window.encodeURIComponent(str);
-  }
-  return '';
-}