Remove 32-bit/i386 macOS build from CI

https://developer.apple.com/documentation/xcode-release-notes/xcode-10-release-notes
The macOS 10.14 SDK no longer contains support for compiling 32-bit applications. If developers need to compile for i386, Xcode 9.4 or earlier is required. (39858111)

No stable GitHub Actions macOS image includes that old of an Xcode
version.

.github/workflows/ci.yml

@@ -86,7 +86,7 @@ jobs:
           - { name: "macOS Big Sur 11", distro: macos-11 }
           - { name: "macOS Monterey 12", distro: macos-12 }
         build_type: [Debug, Release]
-        toolchain: ["gcc-macos-i386", "gcc-macos-x86_64", "gcc-macos-fat"]
+        toolchain: ["gcc-macos-x86_64"]
 
     name: "${{matrix.os.name}} ${{matrix.build_type}} ${{matrix.toolchain}}"
     runs-on: ${{matrix.os.distro}}