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>