MAINTAINERS: remove drivers/mmc/host/imxmmc.*

This driver was removed by commit 6187fee4 ("mmc: remove imxmmc
driver").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>