Revert "[POWERPC] Add compat handler for PTRACE_GETSIGINFO"

This reverts commit e4cc58944c1e2ce41e3079d4eb60c95e7ce04b2b, as
requested by Roland McGrath, because compat_ptrace_request (added in
commit e16b27816462de700f9508d86954410c41105dc2, "ptrace:
compat_ptrace_request siginfo") now handles this case.

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