Remove unused code from Bookmark model

There are a number of functions in app/models/bookmark.rb that were originally used to calculate information about the bookmarkable for use in indexing/search. But now that bookmark search uses BookmarkedWorkIndexer, BookmarkedExternalWorkIndexer, and BookmarkedSeriesIndexer to index information about the bookmarkables, those functions are unused.

The following functions have information about the bookmarkable alone:

• bookmarkable_pseud_names

• bookmarkable_pseud_ids

• bookmarkable_collection_ids

• bookmarkable_title

• bookmarkable_posted

• bookmarkable_restricted

• bookmarkable_hidden

• bookmarkable_complete

• bookmarkable_language_id

(Note that bookmarkable_date should not be deleted, because it's still used by the BookmarkIndexer.)

In addition, the following functions were formerly used to calculate information about the bookmark's tags and the bookmarkable's tags:

• tag

• tag_ids

• filters

• filters_for_facets

• filter_names

• filter_ids

• fandom_ids

• character_ids

• relationship_ids

• freeform_ids

• rating_ids

• warning_ids

• category_ids

All of these functions are currently unused, and should be deleted (to improve code coverage and make it easier to follow the flow of control in the parts of the class that are being used).

Testing Instructions: As long as the PR passes automated tests, I'm not sure any manual testing is required. (Since these functions were only used in indexing, the indexing in the automated tests would fail if any critical functions were missing.) But if you wanted to be thorough, you could try creating a bookmark on a brand new work and ensure it can be found with bookmark search.