types: Remove call_rcu_bh() and call_rcu_sched()

Now that call_rcu()'s callback is not invoked until after bh-disable and
preempt-disable regions of code have completed (in addition to explicitly
marked RCU read-side critical sections), call_rcu() can be used in place
of call_rcu_bh() and call_rcu_sched().  This commit therefore removes
these two API members from the callback_head structure's header comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>