Remove CollectionUtils and StringUtils.htmlEscape()

Working toward #8751's ultimate goal.

* The entire `utils/CollectionUtils` module and `StringUtils.htmlEscape()` function was deprecated by #5539, which was implemented in Sprint 34. The newest reference to CollectionUtils was in src/brackets.js back in January (471a8c1e376607466488aa47fb2bf01108739123), which was Sprint 36, but I find nothing on the wiki explaining how long a deprecation cycle is. There are also no tests for nor core usage  of CollectionUtils.

* `StringUtils.htmlEscape()` has no usage in Brackets core at all.

~~There is some overlap between this and #8954. It might be best for that to be merged first, unless @MarcelGerber wants to make these same changes in that PR.~~