Don't build GtkBackend in Windows GitHub Actions workflow

It doesn't work for some reason, and WinUIBackend is more important for
now anyway.

Author: stackotter

.github/workflows/swift-windows.yml

@@ -48,5 +48,3 @@ jobs:
       run: swift build --target SwiftCrossUI -v
     - name: Build WinUIBackend
       run: swift build --target WinUIBackend
-    - name: Build GtkBackend
-      run: swift build --target GtkBackend