staging/fbtft: Remove flexfb

flexfb was an attempt to write a generic fbtft driver that was abandoned.
All the displays it supports are supported by other fbtft drivers.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20190917171843.10334-3-noralf@tronnes.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>