signal: Remove has_group_leader_pid

After the introduction of exchange_tids has_group_leader_pid is
equivalent to thread_group_leader.  After the last couple of cleanups
has_group_leader_pid has no more callers.

So remove the now unused and redundant has_group_leader_pid.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>