Remove fallback to Web rate limits when Git HTTP limits are disabled

What does this MR do and why?

Contributes to #584113

Problem

The code fallback was added to support the settings migration in 18.7. It's not needed anymore in 18.8.

Solution

Remove code fallback

References

Remove code fallback for web rate limiter Git p... (#584113)

MR acceptance checklist

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Related to #584113