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.

scripts/check-submodule-version.sh

@@ -1,34 +0,0 @@
-#!/bin/bash
-
-ROOT_MODULE="github.com/lightningnetwork/lnd"
-
-# The command 'go list -m all' returns all imports in the following format:
-#   github.com/lightningnetwork/lnd/cert v1.1.0 => ./cert
-# The two cut then first split by spaces and then by slashes to extract the
-# submodule names.
-SUBMODULES="$(go list -m all | grep $ROOT_MODULE/ | cut -d' ' -f1 | cut -d'/' -f4-)"
-BRANCH=$1
-
-for m in $SUBMODULES; do
-  has_changes=0
-  git diff --stat $BRANCH.. | grep -q " $m/" && has_changes=1
-  
-  if [[ $has_changes -eq 1 ]]; then
-    has_bump=0
-    git diff $BRANCH.. -- go.mod | \
-      grep -q "^\+[[:space:]]*$ROOT_MODULE/$m " && has_bump=1
-    
-    if [[ $has_bump -eq 0 ]]; then
-      echo "Submodule '$m' has changes but no version bump in go.mod was found"
-      echo "If you update code in a submodule, you must bump its version in "
-      echo "go.mod to the _next_ version so a tag for that version can be"
-      echo "pushed after merging the PR."
-      exit 1
-    else
-      echo "Submodule '$m' has changes but go.mod bumps it to: "
-      git diff $BRANCH.. -- go.mod | grep $m
-    fi
-  else
-    echo "Submodule '$m' has no changes, skipping"
-  fi
-done