powerpc: Drop orphaned VAS MAINTAINERS entry

The MAINTAINERS entry for VAS (Virtual Accelerator Switchboard) no
longer has any maintainers, it just points to linuxppc-dev, since commit
60496069 ("powerpc: Update MAINTAINERS for ibmvnic and VAS").

So just drop the VAS entry, all the paths are already covered by the
main powerpc entry, ie. the output of get_maintainer.pl is unchanged.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230221101952.2697101-1-mpe@ellerman.id.au