Disable macOS CI checks

Instead of an open source toolchain, the macOS CI build uses
the Xcode-bundled stdlib (or maybe the one in the OS), which
doesn't permit the SPI import that string-processing requires.

Author: natecook1000

.github/workflows/pull_request.yml

@@ -11,9 +11,7 @@ jobs:
     with:
       linux_exclude_swift_versions: '[{"swift_version": "5.9"}, {"swift_version": "5.10"}]'
       enable_windows_checks: false
-      windows_exclude_swift_versions: '[{"swift_version": "5.9"}]'
-      enable_macos_checks: true
-      macos_exclude_xcode_versions: '[{"xcode_version": "16.0"}, {"xcode_version": "16.1"}]'
+      enable_macos_checks: false
   soundness:
     name: Soundness
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main