This PR completely removes kythe/rust/ and tools/rust/ as the extractor and indexer no longer work with newer versions of Rust. All files related to Rust indexer development have been removed including the pre-commit rule.

However, some Rust elements have been kept:

• rules_rust remains and has been updated to the latest version
• The protobuf and protobuf-codegen crates remain and have been updated to the internal 2.* version
• The Rust protobuf toolchain has been refactored based on new definitions in rules_rust

We still need to generate the Rust protobufs for Kythe so that we can import them internally until the 1st part implementation of Rust proto is complete. Afterwards, we should be able to remove the rest of Rust from the codebase.

This should also unblock #5623