Remove workaround for mettle deprecation warning

Author: jimporter

.github/workflows/build.yml

@@ -58,8 +58,7 @@ jobs:
              boost-version: '1.80.0'}
 
           # macOS builds
-          # FIXME: Remove -Wno-deprecated flag.
-          - {os: macos-latest, compiler: clang, flags: '-Wno-deprecated'}
+          - {os: macos-latest, compiler: clang}
 
           # C++20 builds
           - {os: ubuntu-latest, compiler: gcc, version: '13',