R1:93d7b0122252

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.