Because pointer output is now obfuscated, and because what you really
want to know is whether or not the callback lists are empty, this commit
replaces the srcu_data structure's head callback pointer printout with
a single character that is "." is the callback list is empty or "C"
otherwise.

This is the only remaining user of rcu_segcblist_head(), so this
commit also removes this function's definition.  It also turns out that
rcu_segcblist_tail() no longer has any callers, so this commit removes
that function's definition while in the area.  They were both marked
"Interim", and their end has come.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>