fork: Skip setting TIF_SIGPENDING in ptrace_init_task

The code in calculate_sigpending will now handle this so
it is just redundant and possibly a little confusing
to continue setting TIF_SIGPENDING in ptrace_init_task.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>