rcutorture: Remove cbflood facility

Now that the forward-progress code does a full-bore continuous callback
flood lasting multiple seconds, there is little point in also posting a
mere 60,000 callbacks every second or so.  This commit therefore removes
the old cbflood testing.  Over time, it may be desirable to concurrently
do full-bore continuous callback floods on all CPUs simultaneously, but
one dragon at a time.

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