Revert "[POWERPC] Add compat handler for PTRACE_GETSIGINFO"

This reverts commit e4cc5894, as
requested by Roland McGrath, because compat_ptrace_request (added in
commit e16b2781

, "ptrace:
compat_ptrace_request siginfo") now handles this case.

Signed-off-by: Paul Mackerras <paulus@samba.org>