Remove ubuntu-16.04 from workflows (#1257)

GitHub Actions no longer supports Ubuntu 16.04.
https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/

Author: mishina2228

.github/workflows/build.yml

@@ -15,7 +15,6 @@ jobs:
           # https://github.com/brianmario/mysql2/issues/1165
           # - ubuntu-20.04 # focal
           - ubuntu-18.04 # bionic
-          # - ubuntu-16.04 # xenial
         ruby:
           - '3.1'
           - '3.0'
@@ -28,8 +27,6 @@ jobs:
           - '2.1'
         db: ['']
         include:
-          # Allow failure due to Mysql2::Error: Unknown system variable 'session_track_system_variables'.
-          - {os: ubuntu-16.04, ruby: '2.4', db: mariadb10.0, allow-failure: true}
           # Comment out due to ci/setup.sh stucking.
           # - {os: ubuntu-18.04, ruby: 2.4, db: mariadb10.1}
           # Allow failure due to the issue #965, #1165.