[BUG] Filter already set tags from tag list

[mateusz-bak]

Discussed in #712

^{Originally posted by rYR79435 January 7, 2024}
Is your feature request related to a problem? Please describe.
When I assign a tag to a book, a list of existing tags is displayed. That list contains tags which have already been assigned to that book. Given that selecting a tag a second time doesn't add it to the book a second time, it is redundant to display it as a choice.

Describe the solution you'd like
When assigning tags to a book, remove already assigned tags from the list of all existing tags.