CI: remove default Ubuntu build from GitHub Actions

We are already running a very similar Ubuntu build on Travis CI.
The macOS variant of this default build is kept on Github Actions.

.github/workflows/cpp.yml

@@ -1,17 +0,0 @@
-name: Build on Ubuntu with default options
-
-on: [push]
-
-jobs:
-  build:
-
-    runs-on: ubuntu-latest
-    
-    steps:
-    - uses: actions/checkout@v1
-    - name: configure
-      run: ./buildconf && ./configure
-    - name: make
-      run: make
-    - name: make check
-      run: make test-nonflaky