In the olden days we insisted that only TcTyVars could appear
in a TcType.  But now we are more accommodating; see TcType
  Note [TcTyVars and TyVars in the typechecker]

This patch removes a function that converted a Type to a TcType.
It didn't do anything useful except statisfy an invariant that
we no longer have.  Now it's gone.