rcutorture: Remove ->ext_irq_conflict field

Back when there was a separate RCU-bh flavor, the ->ext_irq_conflict
field was used to prevent executing local_bh_enable() while interrupts
were disabled.  However, there is no longer an RCU-bh flavor, so this
commit removes the no-longer-needed ->ext_irq_conflict field.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>