Remove Ubuntu 16 CI runs

Because Github actions doesn't have images for Ubuntu 16 any more.

Author: joto

.github/workflows/ci.yml

@@ -3,54 +3,6 @@ name: CI
 on: [ push, pull_request ]
 
 jobs:
-  ubuntu16-clang6-dev:
-    runs-on: ubuntu-16.04
-    env:
-      CC: clang-6.0
-      CXX: clang++-6.0
-      BUILD_TYPE: Dev
-      WITH_PROJ: ON
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: true
-      - uses: ./.github/actions/install-ubuntu
-      - uses: ./.github/actions/cmake
-      - uses: ./.github/actions/build
-      - uses: ./.github/actions/ctest
-
-  ubuntu16-clang8-dev:
-    runs-on: ubuntu-16.04
-    env:
-      CC: clang-8
-      CXX: clang++-8
-      BUILD_TYPE: Dev
-      WITH_PROJ: ON
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: true
-      - uses: ./.github/actions/install-ubuntu
-      - uses: ./.github/actions/cmake
-      - uses: ./.github/actions/build
-      - uses: ./.github/actions/ctest
-
-  ubuntu16-clang9-dev:
-    runs-on: ubuntu-16.04
-    env:
-      CC: clang-9
-      CXX: clang++-9
-      BUILD_TYPE: Dev
-      WITH_PROJ: ON
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: true
-      - uses: ./.github/actions/install-ubuntu
-      - uses: ./.github/actions/cmake
-      - uses: ./.github/actions/build
-      - uses: ./.github/actions/ctest
-
   ubuntu18-clang9-dev:
     runs-on: ubuntu-18.04
     env:
@@ -117,22 +69,6 @@ jobs:
       - uses: ./.github/actions/build
       - uses: ./.github/actions/ctest
 
-  ubuntu16-gcc5-dev:
-    runs-on: ubuntu-16.04
-    env:
-      CC: gcc-5
-      CXX: g++-5
-      BUILD_TYPE: Dev
-      WITH_PROJ: ON
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: true
-      - uses: ./.github/actions/install-ubuntu
-      - uses: ./.github/actions/cmake
-      - uses: ./.github/actions/build
-      - uses: ./.github/actions/ctest
-
   ubuntu18-gcc7-dev:
     runs-on: ubuntu-18.04
     env: