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.

app/models/post.rb

@@ -30,7 +30,6 @@ class Post < ApplicationRecord
   validate :updater_can_change_rating
   before_save :update_tag_post_counts
   before_save :set_tag_counts
-  after_save :set_pool_category_pseudo_tags
   before_create :autoban
   after_save :create_version
   after_save :update_parent_on_save
@@ -1041,7 +1040,6 @@ class Post < ApplicationRecord
 
       with_lock do
         self.pool_string = "#{pool_string} pool:#{pool.id}".strip
-        set_pool_category_pseudo_tags
         update_column(:pool_string, pool_string) unless new_record?
         pool.add!(self)
       end
@@ -1053,7 +1051,6 @@ class Post < ApplicationRecord
 
       with_lock do
         self.pool_string = pool_string.gsub(/(?:\A| )pool:#{pool.id}(?:\Z| )/, " ").strip
-        set_pool_category_pseudo_tags
         update_column(:pool_string, pool_string) unless new_record?
         pool.remove!(self)
       end
@@ -1064,18 +1061,6 @@ class Post < ApplicationRecord
         pool.remove!(self)
       end
     end
-
-    def set_pool_category_pseudo_tags
-      self.pool_string = (pool_string.split - ["pool:series", "pool:collection"]).join(" ")
-
-      pool_categories = pools.undeleted.pluck(:category)
-      if pool_categories.include?("series")
-        self.pool_string = "#{pool_string} pool:series".strip
-      end
-      if pool_categories.include?("collection")
-        self.pool_string = "#{pool_string} pool:collection".strip
-      end
-    end
   end
 
   module VoteMethods