rcu: Remove unused rcu_dynticks_snap() from Tiny RCU

The rcu_dynticks_snap() function is defined in include/linux/rcutiny.h,
but is no longer used by Tiny RCU.  This commit therefore removes it.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcutiny.h

@@ -27,12 +27,6 @@
 
 #include <linux/ktime.h>
 
-struct rcu_dynticks;
-static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
-{
-	return 0;
-}
-
 /* Never flag non-existent other CPUs! */
 static inline bool rcu_eqs_special_set(int cpu) { return false; }