Remove extra CI testing on older Ubuntu

Testing the older LTS does not require testing the full matrix
to test for compatibility.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

.travis.yml

@@ -21,18 +21,10 @@ env:
 matrix:
   include:
     # Skip testing previous LTS (Xenial / Ubuntu 16.04 LTS) on pull requests
-    - if: type != pull_request
-      os: linux
-      dist: xenial
-      env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runc.v1 TRAVIS_CGO_ENABLED=1 TRAVIS_DISTRO=xenial
     - if: type != pull_request
       os: linux
       dist: xenial
       env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runc.v2 TRAVIS_CGO_ENABLED=1 TRAVIS_DISTRO=xenial
-    - if: type != pull_request
-      os: linux
-      dist: xenial
-      env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runtime.v1.linux TRAVIS_CGO_ENABLED=1 TRAVIS_DISTRO=xenial
 
 go_import_path: github.com/containerd/containerd