[fix] drop useless pylint: disable=undefined-variable

Since 7b235a1c (see line 591) it is no longer needed to disable
'undefined-variable' for names defined in::

   PYLINT_ADDITIONAL_BUILTINS_FOR_ENGINES

Suggested-by: @dalf https://github.com/searxng/searxng/issues/102#issuecomment-914068609


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>