Remove rust toolchain registration

This doesn't alleviate Bazel's problem. rules_rust are still fetched because they register toolchains themselves.

However the change pushes into the direction, where I believe that only the owning module should be concerned with its toolchain registration and the toolchain selection should be controlled via a flag mechanism or similar. Removing the registration now, can alleviate problems in the future, when we figure out how to prevent unnecessary fetches.

PiperOrigin-RevId: 684721071