Remove use of mixed C++/Rust targets in rust_gtest_interop

Rust depending on C++ is banned, so we remove the tests of the
functionality to have Rust use a custom choice of a `testing::Test`
subclass.

The functionality remains in the proc macro and can be tested again if
we want to support Rust depending on C++.

R=lukasza@chromium.org

Bug: 1381552
Change-Id: I992be78d8a88ac16cf322e85a492365d031568c3
Cq-Include-Trybots: luci.chromium.try:linux-rust-x64-dbg,linux-rust-x64-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4006874


Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067729}