Stop triggering Rust CI on C source changes

The Rust code no longer depends on the C source in this repository,
so remove `include/**` and `src/**` from the pull_request path filter
in the Rust workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: soutaro

.github/workflows/rust.yml

@@ -8,8 +8,6 @@ on:
     paths:
       - ".github/workflows/rust.yml"
       - "rust/**"
-      - "include/**"
-      - "src/**"
 
 env:
   RUSTFLAGS: "-D warnings"