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