Remove the logic that was intended to ignore Hypothesis UI when
generating range selectors.

We don't need this any more because all Hypothesis UI elements in the
host page which contain text have `user-select` set to prevent text
selection. This affects the bucket bar, vertical toolbar and adder.

In addition, the logic did not serve its intended purpose as it only applied to
range selector generation. If the user did somehow succeed in selecting text in
the bucket bar for example the client would still generate quote and position
selectors that referred to that content. What the client would need to
do instead is modify the range from which the annotation's selectors
were generated.

As a side effect this fixes a regression introduced in a5540581 which caused
range selector generation to fail in Chrome and Firefox due to use of
`:not(<inner selector>)` selector where `<inner selector>` is a
list of selectors (the `IGNORE_SELECTOR` value from `guest.js`).