MIPS: Remove get_current_pgd().

It is unused in the tree.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3557/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>