R1:b4681aa0c896

Remove dead link to Perforce blog about Git Fusion

Summary: Fixes T9941. I think someone from Perforce emailed us about 10 years ago and I added this link in response, but I haven't seen other interest in Perforce since then. Link is now dead.

Test Plan:
- {nav Diffusion > Create Repository > Import Existing}, no more Perforce link.
- Grepped for `perforce`.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9941

Differential Revision: https://secure.phabricator.com/D14720