Remove 'initFromConduit' option from Diffusion

Summary:
Ref T2783. I think this served two purposes:

• Improving performance in cases where we "know" a repository is local.
• Preventing loops.

It is now obsolete:

• After D11476, refs can almost always resolve on a fast path.
• As T2783 moves forward, we can usually no longer know when a repository is local without actually looking it up -- almost everything is allowed to run anywhere.
• The cluster behavior in D11475 now prevents loops.

Test Plan: grep, browsed around. This didn't really do much of anything anymore.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11477