Can't reproduce the original problem anymore, and in the
meantime the sidebar color button code was totally replaced
with a SvxColorToolBoxControl based solution (see tdf#84844).
So no evidence we still need this special handling of COL_BLACK.

Change-Id: I9ab5e65e6ea1dc599ba850cfcaae62c73947159a
Reviewed-on: https://gerrit.libreoffice.org/43237Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>
Tested-by: Maxim Monastirsky <momonasmon@gmail.com>