Move "Delete Repository" stuff to Diffusion

Summary: Ref T2231. This just moves the "Delete" dialog from Repositories to Diffusion. This dialog just shows instructions and isn't interesting.

Test Plan: {F75093}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2231

Differential Revision: https://secure.phabricator.com/D7412