Revert "explicitly add catkin_lint to the ci pipeline"

This reverts commit 3108a45.

Author: Felix Exner

.travis.yml

@@ -10,8 +10,8 @@ env:
   global:
     - CCACHE_DIR=$HOME/.ccache
   matrix:
-    - ROS_DISTRO="kinetic" ROS_REPO=ros UPSTREAM_WORKSPACE=file CATKIN_LINT=true
-    - ROS_DISTRO="melodic" ROS_REPO=ros UPSTREAM_WORKSPACE=file CATKIN_LINT=true
+    - ROS_DISTRO="kinetic" ROS_REPO=ros UPSTREAM_WORKSPACE=file
+    - ROS_DISTRO="melodic" ROS_REPO=ros UPSTREAM_WORKSPACE=file
     - ROS_DISTRO="kinetic" CLANG_FORMAT_CHECK=file CLANG_FORMAT_VERSION=6.0
 
 install: