powerpc/powernv/vas: Remove a stray line in Makefile

Remove a bogus line from arch/powerpc/platforms/powernv/Makefile that
was added by commit ece4e512 ("powerpc/vas: Export HVWC to debugfs").

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>