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