MIPS: O32/32-bit: Remove outdated comment

A comment in the O32/32-bit system call code is incorrect since commit
46e12c07 ("MIPS: O32 / 32-bit: Always copy 4 stack arguments.").
Remove it.

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7455/


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