.github: removed consistently failing github workflow

The github workflow fails very often due to broken setup or timeouts,
therefore it has been disabled for now.

.github/workflows/main.yml

@@ -1,48 +0,0 @@
-name: CMake
-
-on: [push, pull_request]
-
-jobs:
-  build:
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [ubuntu-latest]
-        BUILD_SHARED_LIBS: ["ON", "OFF"]
-        ENABLE_DTLS: ["ON", "OFF"]
-        ENABLE_TCP: ["ON", "OFF"]
-        DTLS_BACKEND: ["openssl", "gnutls", "mbedtls", "tinydtls"]
-
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: recursive
-      - name: setup (linux)
-        if: startsWith(matrix.os, 'ubuntu')
-        run: |
-          sudo apt-get update && sudo apt-get install libcunit1-dev libmbedtls-dev libgnutls28-dev
-
-      - name: Create Build Environment
-        run: cmake -E make_directory ${{runner.workspace}}/build
-
-      - name: Configure CMake
-        shell: bash
-        working-directory: ${{runner.workspace}}/build
-        run: |
-          cmake $GITHUB_WORKSPACE \
-          -DENABLE_EXAMPLES=ON \
-          -DENABLE_TESTS=ON \
-          -DBUILD_SHARED_LIBS=${{matrix.BUILD_SHARED_LIBS}} \
-          -DENABLE_DTLS=${{matrix.ENABLE_DTLS}} \
-          -DENABLE_TCP=${{matrix.ENABLE_TCP}} \
-          -DDTLS_BACKEND=${{matrix.DTLS_BACKEND}}
-
-      - name: Build
-        working-directory: ${{runner.workspace}}/build
-        shell: bash
-        run: cmake --build .
-    # TODO: add tests to the pipeline
-    # - name: Test
-    #   working-directory: ${{runner.workspace}}/build
-    #   shell: bash
-    #   run: ctest .