versions: remove 'subsequent' version comparison option.

This option was rarely used and what it actually did was usually
difficult to understand.

app/models/artist_commentary_version.rb

@@ -30,11 +30,6 @@ class ArtistCommentaryVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= ArtistCommentaryVersion.where("post_id = ? and updated_at > ?", post_id, updated_at).order("updated_at asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @current ||= ArtistCommentaryVersion.where(post_id: post_id).order("updated_at desc").limit(1).to_a
     @current.first

app/models/artist_version.rb

@@ -40,11 +40,6 @@ class ArtistVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= ArtistVersion.where("artist_id = ? and created_at > ?", artist_id, created_at).order("created_at asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @previous ||= ArtistVersion.where(artist_id: artist_id).order("created_at desc").limit(1).to_a
     @previous.first

app/models/note_version.rb

@@ -19,11 +19,6 @@ class NoteVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= NoteVersion.where("note_id = ? and version > ?", note_id, version).order("updated_at asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @current ||= NoteVersion.where(note_id: note_id).order("updated_at desc").limit(1).to_a
     @current.first

app/models/pool_version.rb

@@ -100,11 +100,6 @@ class PoolVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= PoolVersion.where("pool_id = ? and version > ?", pool_id, version).order("version asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @current ||= PoolVersion.where(pool_id: pool_id).order("version desc").limit(1).to_a
     @current.first

app/models/post_version.rb

@@ -128,11 +128,6 @@ class PostVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= PostVersion.where("post_id = ? and version > ?", post_id, version).order("version asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @current ||= PostVersion.where(post_id: post_id).order("version desc").limit(1).to_a
     @current.first

app/models/wiki_page_version.rb

@@ -29,11 +29,6 @@ class WikiPageVersion < ApplicationRecord
     @previous.first
   end
 
-  def subsequent
-    @subsequent ||= WikiPageVersion.where("wiki_page_id = ? and id > ?", wiki_page_id, id).order("id asc").limit(1).to_a
-    @subsequent.first
-  end
-
   def current
     @current ||= WikiPageVersion.where(wiki_page_id: wiki_page_id).order("id desc").limit(1).to_a
     @current.first