fbdev: remove display subsystem

This four year old subsystem does not have a single in-tree user
not even in staging and as far as I know also none out-of-tree.
I think that justifies removing it which cleans the config up.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: James Simmons <jsimmons@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>