[TASK] Remove TBE_STYLES mainColors from PHP

TYPO3 uses $TBE_STYLES to define certain
variables like background colors in PHP that
can be overridden with extensions.

In order to streamline the rendering of the
backend that should completely done via
LESS / CSS, the corresponding PHP parts are
removed, as they should have no effect
anyway - this is already done via CSS.

Releases: master
Resolves: #62039
Change-Id: I81533c1d2912fc25b96f106974e5cd625b827c90
Reviewed-on: http://review.typo3.org/33168
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>