Remove preload_repo_cache feature flag

Reduces the number of Redis calls in the Projects API

Changelog: performance

config/feature_flags/development/preload_repo_cache.yml

----
-name: preload_repo_cache
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69627
-rollout_issue_url:
-milestone: '14.3'
-type: development
-group: group::project management
-default_enabled: false

lib/api/projects_relation_builder.rb

@@ -27,8 +27,6 @@ module API
       end
 
       def preload_repository_cache(projects_relation)
-        return unless Feature.enabled?(:preload_repo_cache, default_enabled: :yaml)
-
         repositories = repositories_for_preload(projects_relation)
 
         Gitlab::RepositoryCache::Preloader.new(repositories).preload( # rubocop:disable CodeReuse/ActiveRecord