featureflag: Remove reverse-packfile index feature flag
Since last week, we've had the reverse-packfile index feature flag enabled in both production and staging without any adverse effects. We couldn't see any benefits yet, but in theory we should at least see that git commands now require less memory to run in case there's a reverse index available.
Remove the feature flag. There's no need to default-enable it first given that the feature flag was only added to verify that no adverse effects surface.
Closes #3513 (closed)