Remove "repository.create" Conduit API method

Summary:
Ref T10748. This has had many problems for a long time, can't create hosted repositories, can't create cluster repositories, etc. It is obsoleted by diffusion.repository.edit. Remove it.

(Right now diffusion.repository.edit isn't a strict replacement, but it will be as soon as the URI stuff cuts over.)

Test Plan: Grepped for repository.create.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15838