Remove outdated "#TODO: Fix for binds." comment [ci skip]

This comment was added at eaf54865 but already implemented.

activerecord/lib/active_record/relation.rb

@@ -243,7 +243,6 @@ def find_or_initialize_by(attributes, &block)
     # Please see further details in the
     # {Active Record Query Interface guide}[http://guides.rubyonrails.org/active_record_querying.html#running-explain].
     def explain
-      #TODO: Fix for binds.
       exec_explain(collecting_queries_for_explain { exec_queries })
     end