Maintainers.txt: add Sami Mujawar as top-level ArmVirtPkg reviewer

Maintainers.txt: add Sami Mujawar as top-level ArmVirtPkg reviewer

For distributing ArmVirtPkg patch review tasks better, move Sami Mujawar
from the "ArmVirtPkg: Kvmtool" section to the top-level "ArmVirtPkg"
section.

Given that "ArmVirtPkg: Kvmtool" remains without a specific "R" role,
remove "ArmVirtPkg: Kvmtool" altogether.

Cc: Andrew Fish <afish@apple.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Julien Grall <julien@xen.org>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210514114857.12286-1-lersek@redhat.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>