posts: stop using pool_string attribute.

Stop using the pool_string attribute on posts:

* Stop updating it when adding or removing posts from pools.
* Stop returning pool_string in the /posts.json API.
* Stop including the `data-pools` attribute on thumbnails.

The pool_string attribute was used in the past to facilitate pool:X
searches. Posts had a hidden pool_string attribute that contained a list
of every pool the post belonged to. These pools were treated like fake
hidden tags on the post and a search for `pool:X` was treated like a tag
search.

The pool_string has no longer been used for this purpose for a long time
now, and was only maintained for API compatibility purposes. Getting rid
of it eliminates a bunch of legacy cruft relating to adding and removing
posts from pools.

If you need to see which pools a post belongs to, do this:

* https://danbooru.donmai.us/pools.json?search[post_ids_include_any]=318550

The `data-pools` attribute on thumbnails was used by some people to add
custom borders to pooled posts with custom CSS. This will no longer
work. This was already broken because it included things like collection
pools and deleted pools, which you probably didn't want. Use a
userscript to add this attribute back to thumbnails if you need it.

app/models/pool.rb

@@ -10,7 +10,6 @@ class Pool < ApplicationRecord
   validate :updater_can_edit_deleted
   before_validation :normalize_post_ids
   before_validation :normalize_name
-  after_create :synchronize!
   after_save :create_version
 
   deletable
@@ -130,7 +129,7 @@ class Pool < ApplicationRecord
     self.post_ids = version.post_ids
     self.name = version.name
     self.description = version.description
-    synchronize!
+    save!
   end
 
   def contains?(post_id)
@@ -161,7 +160,6 @@ class Pool < ApplicationRecord
 
     with_lock do
       update(post_ids: post_ids + [post.id])
-      post.add_pool!(self, true)
     end
   end
 
@@ -171,7 +169,6 @@ class Pool < ApplicationRecord
     with_lock do
       reload
       update(post_ids: post_ids - [post.id])
-      post.remove_pool!(self)
     end
   end
 
@@ -181,29 +178,6 @@ class Pool < ApplicationRecord
     Post.joins("JOIN (#{pool_posts.to_sql}) pool_posts ON pool_posts.post_id = posts.id").order("pool_posts.pool_index ASC")
   end
 
-  def synchronize
-    post_ids_before = post_ids_before_last_save || post_ids_was
-    added = post_ids - post_ids_before
-    removed = post_ids_before - post_ids
-
-    added.each do |post_id|
-      post = Post.find(post_id)
-      post.add_pool!(self, true)
-    end
-
-    removed.each do |post_id|
-      post = Post.find(post_id)
-      post.remove_pool!(self)
-    end
-
-    normalize_post_ids
-  end
-
-  def synchronize!
-    synchronize
-    save if will_save_change_to_post_ids?
-  end
-
   def post_count
     post_ids.size
   end