Revert "Add GitHub Actions workflow"

This reverts commit 2993b808.

Actions are disabled in archiso's GitHub mirror.

.github/workflows/lint.yml

-# This is a basic workflow to help you get started with Actions
-
-name: CI
-
-# Controls when the action will run. Triggers the workflow on push or pull request
-# events but only for the master branch
-on:
-  push:
-    branches: [ '*' ]
-  pull_request:
-    branches: [ master ]
-
-# A workflow run is made up of one or more jobs that can run sequentially or in parallel
-jobs:
-  # This workflow contains a single job called "build"
-  lint:
-    # The type of runner that the job will run on
-    runs-on: ubuntu-latest
-
-    # Steps represent a sequence of tasks that will be executed as part of the job
-    steps:
-    # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-    - uses: actions/checkout@v2
-
-    # Runs a single command using the runners shell
-    - name: shellcheck
-      run: make check