Remove mod-only user revert system (#4178).

The mass undo system from #4178 replaces this system.

Followup to f2dccf8cf.

app/logical/user_revert.rb

@@ -1,32 +0,0 @@
-# reverts all changes made by a user
-class UserRevert
-  THRESHOLD = 1_000
-  class TooManyChangesError < RuntimeError ; end
-
-  attr_reader :user_id
-
-  def initialize(user_id)
-    @user_id = user_id
-  end
-
-  def process
-    validate!
-    revert_post_changes
-  end
-
-  def validate!
-    if PostArchive.where(updater_id: user_id).count > THRESHOLD
-      raise TooManyChangesError.new("This user has too many changes to be reverted")
-    end
-  end
-
-  def revert_post_changes
-    PostArchive.where(updater_id: user_id).find_each do |x|
-      x.undo!
-    end
-  end
-
-  def self.can_revert?(user)
-    user.post_update_count <= THRESHOLD
-  end
-end