MAINTAINERS: Remove FMC subsystem

Cleanup MAINTAINERS from FMC record since the subsystem was removed.

Cc: Pat Riehecky <riehecky@fnal.gov>
Fixes: 6a80b300

 ("fmc: Delete the FMC subsystem")
Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20190813061547.17847-1-efremov@linux.com
Reviewed-by: Federico Vaga <federico.vaga@cern.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>