Remove unused GitHub Actions scheduled workflow

GitHub Actions' scheduled workflows only run on the `master` branch.
The `master` branch's scheduled workflow is now configured to build
both the `master` and `stable` branches of the documentation.

(cherry picked from commit 85bc5aa296)

.github/workflows/build_offline_docs.yml

@@ -1,31 +0,0 @@
-name: Build documentation for offline usage
-on:
-  schedule:
-    # Every week on Monday at midnight (UTC).
-    # This keeps the generated HTML documentation fresh.
-    - cron: '0 0 * * 1'
-
-jobs:
-  build:
-    runs-on: ubuntu-20.04
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v2
-
-      - name: Install dependencies
-        run: |
-          sudo pip3 install -r requirements.txt
-          sudo pip3 install codespell
-
-      # Build the HTML to upload it.
-      - name: Sphinx build
-        run: |
-          sphinx-build --color -d _build/doctrees -W . _build/html
-
-      - uses: actions/upload-artifact@v2
-        with:
-          name: godot-docs-html
-          path: _build/html
-          # Keep the current build and the previous build (in case a scheduled build failed).
-          # This makes it more likely to have at least one successful build available at all times.
-          retention-days: 15