rcu: Remove unneeded rcu_process_callbacks() declarations

The declarations of __rcu_process_callbacks() and rcu_process_callbacks()
are not needed, as the definition of both of these functions appear before
any uses.  This commit therefore removes both declarations.

Reported-by: "Ahmed, Iftekhar" <ahmedi@oregonstate.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>