powerpc/mm: unstub radix__vmemmap_remove_mapping()

Use remove_pagetable() and friends for radix vmemmap removal.

We do not require the special-case handling of vmemmap done in the x86
versions of these functions. This is because vmemmap_free() has already
freed the mapped pages, and calls us with an aligned address range.

So, add a few failsafe WARNs, but otherwise the code to remove physical
mappings is already sufficient for vmemmap.

Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>