efi: Remove Matthew Garrett as efivarfs maintainer

Matthew Garrett is still listed as a efivarfs co-maintainer, but the
email address bounces, and Matt is no longer involved in maintaining
this code.

So let's remove Matt as a efivarfs co-maintainer from MAINTAINERS.
Thanks for all the hard work!

Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

MAINTAINERS

@@ -7616,7 +7616,6 @@ S:	Maintained
 F:	drivers/firmware/efi/test/
 
 EFI VARIABLE FILESYSTEM
-M:	Matthew Garrett <matthew.garrett@nebula.com>
 M:	Jeremy Kerr <jk@ozlabs.org>
 M:	Ard Biesheuvel <ardb@kernel.org>
 L:	linux-efi@vger.kernel.org