Kill off ifaceTyVarsOfType

IfaceTypes are really not well suited to finding free variables etc.
Nevertheless, there was quite a lot of code to do just that; but it
was only used to see if a kind is variable-free so as to decide
whether to print a forall binder.

This patch simplifies to deal with just that case, replacing all
the free-vars stuff with just ifTypeIsVarFree