rcu/tree_plugin: Don't handle the case of 'all' CPU range

The 'all' semantics is now supported by the bitmap_parselist() so we can
drop supporting it as a special case in RCU code.  Since 'all' is properly
supported in core bitmap code, also drop legacy comment in RCU for it.

This patch does not make any functional changes for existing users.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>