powerpc: kill the obsolete code under is_global_init()

The code under "if (is_global_init())" is bogus, and is_global_init()
itself is not right in mt case.

Contrary to what the comment says, nowadays force_sig_info() does kill
init even if the handler is SIG_DFL. Note that force_sig_info() clears
SIGNAL_UNKILLABLE exactly for this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>