and drop unused 4BIT_TRANS and 1BIT_MATRIX enumerators

Also fix a bug in x11::convertBitmapDepth where we were incorrectly
passing BmpConversion enumeratirs to ReduceColors

Reviewed-on: https://gerrit.libreoffice.org/34062Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
(cherry picked from commit b5699cd0)

Change-Id: I903c6866750e46ee752e10a17c05fcaaf6b11242
(cherry picked from commit 2d171734)