Remove invalid comment

This is not valid anymore after 08477a6

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -279,10 +279,6 @@ def client_encoding
       end
 
       def exec_query(sql, name = 'SQL', binds = [])
-        # If the configuration sets prepared_statements:false, binds will
-        # always be empty, since the bind variables will have been already
-        # substituted and removed from binds by BindVisitor, so this will
-        # effectively disable prepared statement usage completely.
         if without_prepared_statement?(binds)
           result_set, affected_rows = exec_without_stmt(sql, name)
         else