resourceloader: Remove redundant getModifiedTime implementations

resourceloader: Remove redundant getModifiedTime implementations

Follows-up f37cee996e which replaced the getHashMtime() and
getDefinitionMtime() methods with dummies that always return 1.

These getModifiedTime() implementations were only tracking the
definition summary or custom hash, which is already tracked
by getVersionHash().

Notes:
* SpecialCharacterDataModule: This one was odd as it was tracking
  both the mtime *and* the file contents.

* UserCSSPrefsModule/UserOptionsModule: Remove redundant caching.
  Already taken care of by getVersionHash() as of f37cee996e.

Bug: T94074
Change-Id: I6e37c3c2f85ef4599a8643b0efabc18de2f51ec4