Simplify TcSimplify, by removing Free

For a long time TcSimplify used a three-way classification of constraints,
into 	Free
	Irred
	ReduceMe
(see the data type WhatToDo).  In the new world of implication constraints,
the Free case does not make so much sense, and I managed to elminate it
altogether, thus simplifying the story somewhat.  Now WhatToDo has constructors
	Stop
	ReduceMe

There should be no change in behaviour.