Remove the identity-substitution "optimisation" from zip_ty_env.

	-- There used to be a special case for when
	--	ty == TyVarTy tv
	-- (a not-uncommon case) in which case the substitution was dropped.
	-- But the type-tidier changes the print-name of a type variable without
	-- changing the unique, and that led to a bug.   Why?  Pre-tidying, we had
	-- a type {Foo t}, where Foo is a one-method class.  So Foo is really a newtype.
	-- And it happened that t was the type variable of the class.  Post-tiding,
	-- it got turned into {Foo t2}.  The ext-core printer expanded this using
	-- sourceTypeRep, but that said "Oh, t == t2" because they have the same unique,
	-- and so generated a rep type mentioning t not t2.
	--
	-- Simplest fix is to nuke the "optimisation"