remove miri-rustc-tests

[RalfJung]

@oli-obk I am not sure if anyone used this program in the last few years. There are no instructions for how to use it. So I think keeping it working is just a waste of time. It now fails to build:

error[E0046]: not all trait items implemented, missing: `visit_foreign_item`
  --> src/bin/miri-rustc-tests.rs:36:17
   |
36 |                 impl<'tcx, 'hir> itemlikevisit::ItemLikeVisitor<'hir> for Visitor<'tcx> {
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `visit_foreign_item` in implementation

So I propose instead of me trying to figure out how to fix this, we just drop this ancient code.

[oli-obk]

@bors r+

yea, I don't plan on reviving it

[bors]

📌 Commit 6145709 has been approved by oli-obk

[bors]

⌛ Testing commit 6145709 with merge 47acece...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 47acece to master...