Remove checkout and update git submodules

Author: MaksimZhukov

.github/workflows/releases-validation.yml

@@ -16,9 +16,6 @@ jobs:
         os: [macos-latest, windows-latest, ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
         python: [3.5.4, 3.6.7, 3.7.5, 3.8.1]
     steps:
-    - name: Checkout
-      uses: actions/checkout@v2
-
     - name: setup-python ${{ matrix.python }}
       uses: actions/setup-python@v2
       with: