Remove shouldCreateDiffusionRequest from Diffusion conduit methods

Summary:
Ref T2683. This has no callsites, and the functionality is covered by the initFromConduit flag.

This simplifies the code and reduces then number of internal diffusion.resolverefs calls we make on, e.g., the Git repository page from 7 to 2.

Test Plan: Grepped for these symbols.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9090