Remove dead code left over from the Rust migration

[Phlabry]

Describe the bug
Several files are unreachable, uncompiled, or reference symbols that no longer exist. #103 asked for the old threading tools and TCP client and server to be removed, and #145 listed "no unused or dead code remains" as an acceptance criterion. Both are only partly done.

Confirmed dead:

• xplane_plugin/include/plugin.h declares elevationMslRef, elevationAglRef, airspeedRef and verticalVelocityRef. None exist in either source file. Nothing includes the header.
• xplane_plugin/include/threading-tools.h declares ThreadQueue, generate_packet and output_xml with no definitions anywhere. Its only includer is a test that shadows it, see gtest target tests a locally redefined function, not production code #191.
• xplane_plugin/subprojects/baton/main.cpp calls new_wrapper(), which no longer exists; the bridge exports new_baton_handle(). It is not referenced by the baton meson build, so it is never compiled and would fail if it were.
• src/clientserver declares binaries at src/server.rs and src/client.rs. Neither file exists, so the crate cannot build.
• relay/src/ipc.rs is unreferenced (mod ipc; is commented out at relay/src/main.rs:2) and would not compile if it were, because it imports crate::IpcThreadMessage, a type that does not exist.
• src/client.rs at the repository root is orphaned. No manifest points at it.
• xplane_plugin/tests/test.cc, headingtests.cpp, headingtests.hpp and CMakeLists.txt are tracked but referenced by no build.

Expected behavior
These are deleted, finishing #103 and #145.

Additional context
Deleting threading-tools.h requires removing the include at xplane_plugin/tests/test_threading_tools.cpp:1. Best done alongside the testing issue.

Found while writing the setup documentation in #138 / #178.