rcu: Remove obsolete callback-invocation statistics for debugfs

The debugfs interface displayed statistics on RCU callback invocation but
this interface has since been removed.  This commit therefore removes the
no-longer-used rcu_data structure's ->n_cbs_invoked and ->n_nocbs_invoked
fields along with their updates.

If this information proves necessary in the future, the corresponding
event traces will be added.

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