docs+github+scripts: remove submodule update check

Because we want to avoid local replace directives (as they make it easy
to screw up things for external applications that use lnd as a library
without us noticing), we're going to switch over to a new process.
See section "Use of Golang submodules" in the code contribution
guideline.

.github/workflows/main.yml

@@ -104,34 +104,6 @@ jobs:
       - name: check commits
         run: scripts/check-each-commit.sh upstream/master
 
-  ########################
-  # check submodules
-  ########################
-  check-submodules:
-    name: check submodules
-    runs-on: ubuntu-latest
-    steps:
-      - name: git checkout
-        uses: actions/checkout@v2
-        with:
-          fetch-depth: 0
-
-      - name: setup go ${{ env.GO_VERSION }}
-        uses: actions/setup-go@v2
-        with:
-          go-version: '${{ env.GO_VERSION }}'
-
-      - name: fetch and rebase on master
-        run: |
-          git remote add upstream https://github.com/lightningnetwork/lnd
-          git fetch upstream
-          export GIT_COMMITTER_EMAIL="lnd-ci@example.com"
-          export GIT_COMMITTER_NAME="LND CI"
-          git rebase upstream/master
-
-      - name: check submodules
-        run: scripts/check-submodule-version.sh upstream/master
-
   ########################
   # lint code
   ########################