task: get rid of the insert_task method

Nothing calls it (not even tests), and there's significant functional
overlap with `try_lock`.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/task.h

@@ -172,9 +172,6 @@ namespace crucible {
 		/// objects it holds, and exit its Task function.
 		ExclusionLock try_lock(const Task &task);
 
-		/// Execute Task when Exclusion is unlocked (possibly
-		/// immediately).
-		void insert_task(const Task &t);
 	};
 
 	/// Wrapper around pthread_setname_np which handles length limits