Remove uncalled DiffusionRequest->getCallsign()
Summary: Ref T4245. This has no callers.
Test Plan:
- Ran git grep -i 'getCallsign(' and visually verified that no callers could reasonably be DiffusionRequest objects (there are only 23 remaining sites, and about half are $this->... in PhabricatorRepository.
- Browsed around directory/file/branch/content/diff/etc pages in Diffusion.
Reviewers: chad, avivey
Reviewed By: avivey
Maniphest Tasks: T4245
Differential Revision: https://secure.phabricator.com/D15295