MIPS: Remove unused sys_32_mmap2

The sys_32_mmap2 function has been unused since we started using syscall
wrappers in commit dbda6ac0 ("MIPS: CVE-2009-0029: Enable syscall
wrappers."), and is indeed identical to the sys_mips_mmap2 function that
replaced it in sys32_call_table.

Remove the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20107/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org