pools: stop maintaining pool category pseudotags in pool strings (#4160)

Stop maintaining pool category pseudo tags (pool:series, pool:collection)
in pool strings. They're no longer used and the changes to the
`Post#pools` method in dc4d2e54b caused issues with this.

Also allow Members to change the category of large pools again. This was
only restricted because maintaining these pseudotags forced us to update
every post in the pool whenever a pool's category was changed.

config/danbooru_default_config.rb

@@ -103,11 +103,6 @@ module Danbooru
       2
     end
 
-    # Members cannot change the category of pools with more than this many posts.
-    def pool_category_change_limit
-      100
-    end
-
     # Users cannot search for more than X regular tags at a time.
     def base_tag_query_limit
       6