powerpc: implement vmemmap_remove_mapping() for BOOK3S

This is to be called in vmemmap_free(), leave the implementation on BOOK3E
empty as before.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>