MAINTAINERS: Remove Matt Fleming as EFI co-maintainer

Instate Ard Biesheuvel as the sole EFI maintainer and leave other folks
as maintainers for the EFI test driver and efivarfs file system.

Also add Ard Biesheuvel as the EFI test driver and efivarfs maintainer.

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ivan Hu <ivan.hu@canonical.com>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20180103094417.6353-1-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>