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:

• 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