Remove unused tag filter

This filter was added in commit 4285ba4b, it was changed in commit
002d8688, and most of the code from the original commit has disappeared
without a trace (maybe due to a merge conflict?).

This filter could actually be useful if we started using it when users
click on a tag. Since we don't, I'm removing it. We might add it back if
we decide to actually use it.

app/controllers/debates_controller.rb

@@ -4,7 +4,6 @@ class DebatesController < ApplicationController
   include FlagActions
   include Translatable
 
-  before_action :parse_tag_filter, only: :index
   before_action :authenticate_user!, except: [:index, :show, :map]
   before_action :set_view, only: :index
   before_action :debates_recommendations, only: :index, if: :current_user