[BUGFIX] Remove itemsProcFunc user_MediaType reference

With removing DAM support this itemsProcFunc line is
somehow left behind.

Remove this line so media elements work again in backend.

Change-Id: I908daf2f19c705bdb435f3a64c79b9c7ccf84512
Reviewed-on: https://review.typo3.org/29925
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer