Delete GitHub-specific files

After migration to Codeberg, GitHub's CI definitions aren't needed any
more. The project will be migrated to Codeberg CI when it exits private
beta.

Also delete banner image file, since it wasn't being used anywhere.

.github/workflows/ci.yml

@@ -1,63 +0,0 @@
-name: Node.js CI
-
-on:
-  push:
-    branches:
-      - main
-      - v*
-  pull_request:
-    branches:
-      - main
-
-jobs:
-  lint:
-    name: Lint package
-    runs-on: ubuntu-latest
-
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-      - name: Setup Node
-        uses: actions/setup-node@v2
-        with:
-          node-version: "16"
-      - uses: pnpm/action-setup@v2.0.1
-        with:
-          version: 6.0.2
-      - name: Install dependencies
-        run: pnpm i
-      - name: Lint
-        run: pnpm run lint
-
-  build-docs:
-    name: Build and test package and deploy docs
-    runs-on: ubuntu-latest
-
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-      - name: Setup Node
-        uses: actions/setup-node@v2
-        with:
-          node-version: "16"
-      - uses: pnpm/action-setup@v2.0.1
-        with:
-          version: 6.0.2
-      - name: Install dependencies
-        run: pnpm i
-      - name: Test package
-        run: pnpm run test
-
-      - name: Copy built files to docs
-        run: cp ./dist/shareon.min.* ./docs
-        if: ${{ github.ref == 'refs/heads/main' }}
-      - name: Deploy to GitHub Pages
-        uses: crazy-max/ghaction-github-pages@v2
-        with:
-          keep_history: true
-          build_dir: docs
-          fqdn: shareon.js.org
-          jekyll: false
-        if: ${{ github.ref == 'refs/heads/main' }}
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}