powerpc/mm: drop function __ioremap()

__ioremap() is not used anymore, drop it.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ccc439f481a0884e00a6be1bab44bab2a4477fea.1566309262.git.christophe.leroy@c-s.fr