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