MAINTAINERS: Remove deleted platform efm32

There are no files left to be maintained, to remove the maintainer
entry, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210115155130.185010-8-u.kleine-koenig@pengutronix.de'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>