promote Perforce from `experimentalFeatures`, step 3

[peterguy]

If Perforce is going to become a first-class citizen of Sourcegraph, it needs to graduate from being an experimental feature.

First step is for the system to treat experimentalFeatures.perforce as "enabled" by default, meaning that unless it's explicitly "disabled", it's enabled and active. #51903

Then a cleanup OOB migration can be devised that removes that setting from the site configuration. #52047

Finally, that setting can be removed from the schema, and from everywhere that checks it.

This issue is for the third step - remove settings.