ci: remove x86_64 macOS

GitHub is removing the free-tier of x86_64 macOS in September
and Rust is deprecating `x86_64-apple-darwin` to tier 2 with host tools.
This job is also usually the slowest, so it's a bottleneck as well.

Author: kkysen

.github/workflows/ci.yml

@@ -18,10 +18,6 @@ jobs:
             os: Linux
             arch: x86_64
             clang-version: 15
-          - runner: macos-13
-            os: macOS
-            arch: x86_64
-            clang-version: 17
           - runner: macos-15
             os: macOS
             arch: aarch64