Stop building with Xcode 16.4

Co-authored-by: sashaweiss-signal <sasha@signal.org>

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
     strategy:
       matrix:
         # Add additional Xcode versions here if necessary.
-        xcode: ["Xcode_26.0", "Xcode_16.4"]
+        xcode: ["Xcode_26.0"]
 
     steps:
       - name: Set Xcode version
@@ -41,12 +41,10 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Check Xcode version
-        if: ${{ matrix.xcode != 'Xcode_16.4' }}
         run: |
           Scripts/check_xcode_version.py --relaxed
 
       - name: Download Metal toolchain
-        if: ${{ matrix.xcode == 'Xcode_26.0' }}
         run: |
           xcodebuild -downloadComponent MetalToolchain