Remove `tls::with_related_context`.

This function gets the current `ImplicitCtxt` and checks that its `tcx`
matches the passed-in `tcx`. It's an extra bit of sanity checking: when
you already have a `tcx`, and you need access to the non-`tcx` parts of
`ImplicitCtxt`, check that your `tcx` matches the one in `ImplicitCtxt`.

However, it's only used in two places: `start_query` and
`current_query_job`. The non-checked alternatives (`with_context`,
`with_context_opt`) are used in more places, including some where a
`tcx` is available. And things would have to go catastrophically wrong
for the check to fail -- e.g. if we somehow end up with multiple
`TyCtxt`s. In my opinion it's just an extra case to understand in
`tls.rs` that adds little value.

This commit removes it. This avoids the need for `tcx` parameters in a
couple of places. The commit also adjusts how `start_query` sets up its
`ImplicitCtxt` to more closely match how similar functions do it, i.e.
with `..icx.clone()` for the unchanged fields.