powerpc: Remove ancient DEBUG_SIG code

We have some compile-time disabled debug code in signal_xx.c. It's from
some ancient time BG, almost certainly part of the original port, given
the very similar code on other arches.

The show_unhandled_signal logic, added in d0c3d534a438 (2.6.24) is
cleaner and prints more useful information, so drop the debug code.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>