fbdev: remove mbx framebuffer driver

The only in-tree user for mbx driver for Intel 2700G graphics chip was
cm-x270 platform. Since this platform was removed by the commit
9d323914 ("ARM: pxa: remove Compulab pxa2xx boards") there is no
point to keep the obsolete framebuffer driver.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200830115524.431278-1-rppt@kernel.org