Accept github hostkey, remove submodule

Summary:
- Accept github host key for ssh automatically
- Nuke senza submodule since it is no longer used

Test Plan: - Ran a deploy a few minutes ago.

Reviewers: #haskell.org_infrastructure, austin

Reviewed By: #haskell.org_infrastructure, austin

Subscribers: austin

Differential Revision: https://phabricator.haskell.org/D256