MAINTAINERS: Remove non-existent VAS file

The file arch/powerpc/include/uapi/asm/vas.h was considered but
never merged and should be removed from the MAINTAINERS file.

While here, add missing email address.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>