[TASK] Remove GFX/im_combine_filename setting

Image Magick support for versions 4 and 5 was dropped. The setting
to differentiate combine from composite is obsolete now and composite
is used always.

Change-Id: I0fbc847167d340fd83315285745c63d53225891a
Resolves: #52011
Releases: 6.2
Reviewed-on: https://review.typo3.org/23836
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn