MIPS: Remove unused code from arch/mips/kernel/syscall.c

The variable arg3 in _sys_sysmips() is unused.  Remove it.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2034/


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