Remove macOS/iOS builds.

It's too hard to get them working on modern GH Actions runners.

.github/workflows/build.yml

@@ -56,29 +56,9 @@ jobs:
       unreal-program-name: "UE_5.1"
       upload-package-base-name: "CesiumForUnreal-51-linux"
       clang-version: "v20_clang-13.0.1-centos7"
-  macOS51:
-    uses: ./.github/workflows/buildMac.yml
-    secrets: inherit
-    with:
-      runner-label: macos-13
-      unreal-engine-version: "5.1.0"
-      unreal-engine-zip: "s3://cesium-unreal-engine/5.1.0/macOS/UE_51_macOS.zip"
-      unreal-program-name: "UE_5.1"
-      upload-package-base-name: "CesiumForUnreal-51-macos"
-      xcode-version: "14.3.1"
-  iOS51:
-    uses: ./.github/workflows/buildiOS.yml
-    secrets: inherit
-    with:
-      runner-label: macos-13
-      unreal-engine-version: "5.1.0"
-      unreal-engine-zip: "s3://cesium-unreal-engine/5.1.0/macOS/UE_51_macOS.zip"
-      unreal-program-name: "UE_5.1"
-      upload-package-base-name: "CesiumForUnreal-51-ios"
-      xcode-version: "14.3.1"
   Combine51:
     runs-on: ubuntu-latest
-    needs: [Windows51, Linux51, Android51, macOS51, iOS51]
+    needs: [Windows51, Linux51, Android51]
     steps:
       - name: Check out repository code
         uses: actions/checkout@v4
@@ -89,16 +69,6 @@ jobs:
           # Make these available to subsequent steps
           echo "CESIUM_UNREAL_VERSION=$CESIUM_UNREAL_VERSION" >> $GITHUB_ENV
           echo "BUILD_CESIUM_UNREAL_PACKAGE_NAME=$BUILD_CESIUM_UNREAL_PACKAGE_NAME" >> $GITHUB_ENV
-      - name: Download iOS build
-        uses: actions/download-artifact@v4
-        with:
-          name: CesiumForUnreal-51-ios-${{ env.CESIUM_UNREAL_VERSION}}
-          path: combine
-      - name: Download macOS build
-        uses: actions/download-artifact@v4
-        with:
-          name: CesiumForUnreal-51-macos-${{ env.CESIUM_UNREAL_VERSION}}
-          path: combine
       - name: Download Android build
         uses: actions/download-artifact@v4
         with: