Remove deprecated ubuntu-16.04 test targets.

Github has deprecated ubuntu-16.04 and it will be removed on 20
September.

.github/workflows/c-cpp.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         # First we test all OSes in the default configuration.
-        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macos-10.15, macos-11.0]
+        os: [ubuntu-20.04, ubuntu-18.04, macos-10.15, macos-11.0]
         configs: [default]
         # Then we include any extra configs we want to test for specific VMs.
         # Valgrind slows things down quite a bit, so start them first.
@@ -47,8 +47,6 @@ jobs:
           - { os: ubuntu-18.04, configs: selinux }
           - { os: ubuntu-18.04, configs: kitchensink }
           - { os: ubuntu-18.04, configs: without-openssl }
-          - { os: ubuntu-16.04, configs: pam }
-          - { os: ubuntu-16.04, configs: kitchensink }
           - { os: macos-10.15,  configs: pam }
           - { os: macos-11.0,   configs: pam }
     runs-on: ${{ matrix.os }}