Remove skip_cleanup (deprecated)

skip_cleanup is now deprecated, and cleanup is false by default.

https://blog.travis-ci.com/2019-08-27-deployment-tooling-dpl-v2-preview-release#cleaning-up-the-git-working-directory

.travis.yml

@@ -38,7 +38,6 @@ deploy:
   api_key:
     secure: mF9U4mO+B+lJKqK6rqom/nOlsLHDRlBVgVIQxUuO4xCPuygD1eer8Hfkvqo58tpsPWF24Iqek4NyFTZ7dY78vmhVUPCxS/nq/+6A0Dyg3uqFYK5F+Sn60e46ZATSOJVDlKTr62ZR04dq0tjrFBwXfNXqv9RMQGjaOVD/U+tKTsw=
   file: wp_launch_check.phar
-  skip_cleanup: true
   on:
     repo: pantheon-systems/wp_launch_check
     tags: true