MIPS: Remove #if 0 r4k_update_mmu_cache_hwbug

The function is #if 0ed out.  There are no other occurrences of its
name in the tree.  It is safe to remove.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/936/


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