signal: Remove no longer required irqsave/restore

Commit a841796f ("signal: align __lock_task_sighand() irq disabling and
RCU") introduced a rcu read side critical section with interrupts
disabled. The changelog suggested that a better long-term fix would be "to
make rt_mutex_unlock() disable irqs when acquiring the rt_mutex structure's
->wait_lock".

This long-term fix has been made in commit b4abf910 ("rtmutex: Make
wait_lock irq safe") for a different reason.

Therefore revert commit a841796f ("signal: align >
__lock_task_sighand() irq disabling and RCU") as the interrupt disable
dance is not longer required.

The change was tested on the base of b4abf910

 ("rtmutex: Make wait_lock
irq safe") with a four hour run of rcutorture scenario TREE03 with lockdep
enabled as suggested by Paul McKenney.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: bigeasy@linutronix.de
Link: https://lkml.kernel.org/r/20180525090507.22248-3-anna-maria@linutronix.de