Delete "$message_type" special case from `with_json_contains_unordered`

[dtolnay]

#13016 added the following hack to accommodate rust-lang/rust#115691, because for now, some compilers supported by Cargo emit "$message_type" in their diagnostics and some do not.

cargo/crates/cargo-test-support/src/compare.rs

Lines 597 to 607 in 71cd3a9

	// Compilers older than 1.76 do not produce $message_type.
	// Treat it as optional for now.
	let mut expected_entries_without_message_type;
	let expected_entries: &mut dyn Iterator<Item = _> =
	if l.contains_key("$message_type") && !r.contains_key("$message_type") {
	expected_entries_without_message_type =
	expected_entries.filter(|entry| entry.0 != "$message_type");
	&mut expected_entries_without_message_type
	} else {
	&mut expected_entries
	};

As soon as Cargo's MSRV increases past 1.76, this should be deleted.

According to @epage, the JSON testing infrastructure is intended to be independent of rustc in the long term. Compiler-specific special cases run counter to this effort to switch to a generic third-party library.

Of course deleting the special case only defers the issue until some other JSON change happens in a future release. We still need to figure out how the generic tool should accommodate this in a principled way.