[MIPS] signal32: remove duplicate code

There's no point for signal32.c to redefine get_sigframe().
It should use the one define in signal.c instead.

The same stands for install_sigtramp().

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>