Remove front::intrinsic::get_tydesc

[catamorphism]

As per FIXME: "remove this when the interface has settled and the
// version in sys is no longer present." The version in sys is still used, so this can't be removed yet.

[graydon]

See also #3475

[luqmana]

So is it that neither is one wanted or could you keep only the one in sys and remove all uses of intrinsic::TyDesc in favour of sys::TypeDesc. As well as move the TyVisitor to core::sys which basically gets rid of librustc/front/intrinsic.rs altogether.

[catamorphism]

Doesn't seem critical for 0.6. De-milestoning

[emberian]

Seems to have been fixed. The fixme is certainly gone.