Remove the code related to FF simplify_logic_to_find_search_proxy_class

Since the FF simplify_logic_to_find_search_proxy_class has been rolled
out completely and is stable so we can remove the code related to this
feature_flag. ProjectWikiProxy will be replaced by WikiProxy.

Changelog: other
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124236
EE: true