rcu: Remove unused function hlist_bl_del_init_rcu()

The function hlist_bl_del_init_rcu() is declared in rculist_bl.h,
but never used.  This commit therefore removes it.

Signed-off-by: Ethan Hansen <1ethanhansen@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>