Cleanup unused rules

[blag]

This repo has accumulated a few rules that haven't ever been enforced (and their actions have never been run) on our existing st2cicd server. I'm considering these to be unused:

• st2cd_slack_e2e_test
• st2cd_slack_pkg_rhel6
• st2cd_slack_pkg_ubuntu16
• st2cd_slack_pytests
• st2cd_slack_upgrade_ubuntu16
• st2cd_slack_webui_pkg
• st2contrib_regenerate_readme

These should be removed so we don't have to continue to spend developer time keeping them around and theoretically operational.