btrfs: remove unused rcu-string printk helpers

The RCU-string API has never taken off and we don't use the printk
helpers provided as we do the protection in our helpers. Remove the "in
RCU" wrappers.

Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>