Update nightly.yml

Author: lahirumaramba

.github/workflows/nightly.yml

@@ -41,11 +41,6 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: 2.1.x
-        
-    - name: Setup .NET Core 3.1
-      uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: 3.1.x
 
     - name: Build with dotnet
       run: dotnet build FirebaseAdmin/FirebaseAdmin