remove docs linter step on travis

this is already done on Jenkins CI, so no need to double our efforts

Signed-off-by: Benjamin Wohlwend <[email protected]>

Author: beniwohli

.travis.yml

@@ -28,10 +28,6 @@ matrix:
     python: 3.6
     script: pip install isort && isort -c -df && echo "OK"
 
-  - stage: linters
-    script: make docs
-    perl: "5.26"
-
   - stage: build
     if: tag =~ ^v\d # only build if this is a version-tag
     dist: trusty