signal: Remove SEND_SIG_FORCED

There are no more users of SEND_SIG_FORCED so it may be safely removed.

Remove the definition of SEND_SIG_FORCED, it's use in is_si_special,
it's use in TP_STORE_SIGINFO, and it's use in __send_signal as without
any users the uses of SEND_SIG_FORCED are now unncessary.

This makes the code simpler, easier to understand and use.  Users of
signal sending functions now no longer need to ask themselves do I
need to use SEND_SIG_FORCED.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>