MAINTAINERS: remove section HISILICON STAGING DRIVERS FOR HIKEY 960/970

MAINTAINERS: remove section HISILICON STAGING DRIVERS FOR HIKEY 960/970

Commit 9bd9e0de1cf5 ("mfd: hi6421-spmi-pmic: move driver from staging")
moves the last driver out of ./drivers/staging/hikey9xx/ and removes that
directory, but missed to adjust the HISILICON STAGING DRIVERS FOR HIKEY
960/970 section in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/staging/hikey9xx/

As the directory ./drivers/staging/hikey9xx/ is gone, remove the section
HISILICON STAGING DRIVERS FOR HIKEY 960/970 in MAINTAINERS as well.

Acked-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210727093154.553-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>