fbdev: remove obsolete CONFIG_FB_SOFT_CURSOR

The config Option FB_SOFT_BUFFER was removed in
c465e05a ("fbcon/fbdev: Move softcursor
out of fbdev to fbcon").

While moving to fbcon this single driver has it left as a select in
KConfig / #ifdef in source.  This last occurence is removed here so the
option is really gone

Signed-off-by: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>