Remove old CI workflow

No longer relevant or needed

Contributes to CURA-9365

.github/workflows/ci.yml

@@ -1,21 +0,0 @@
----
-name: CI
-on:
-  push:
-    branches:
-      - master
-      - 'WIP**'
-      - '4.*'
-      - 'CURA-*'
-  pull_request:
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    container: ultimaker/cura-build-environment
-    steps:
-    - name: Checkout Cura
-      uses: actions/checkout@v2
-    - name: Build
-      run: docker/build.sh
-    - name: Test
-      run: docker/test.sh