ci/github: Ubuntu 18.04 deprecated

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

.github/workflows/build.yml

@@ -31,15 +31,3 @@ jobs:
       run: |
         sudo apt-get install libdbus-1-dev libasound2-dev
         make
-
-  ubuntu_18_04:
-    runs-on: ubuntu-18.04
-    steps:
-    - uses: actions/checkout@v2
-      with:
-        submodules: recursive
-    - name: Build ladi-a2jmidi
-      shell: bash
-      run: |
-        sudo apt-get install libdbus-1-dev libasound2-dev
-        make