ci: Remove the alping compilation test

This workflow has never had a particularly good signal-to-noise ratio,
and disabling it saves us a couple of GH Actions cycles, with only
minor reduction in test reach. We should rather rethink this and use
the installation instructions to write Dockerfiles for each described
architecture, and then have the CI test-build those Dockerfiles. That
would also cover the docs during testing, rather than having yet another
place where the instructions can bitrot away.

Changelog-None No user-visible change.

.github/workflows/ci_build.yml

@@ -1,18 +0,0 @@
-name: CI Compilation testing
-
-on: [push, pull_request]
-
-jobs:
-  test:
-    runs-on: ubuntu-latest
-    timeout-minutes: 120
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - { OS: alpine }
-    steps:
-      - uses: actions/checkout@v2
-      - name: Integration testing
-        run: |
-          docker build -f contrib/docker/Dockerfile.${{matrix.OS}} -t clightning-${{matrix.OS}} .