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. MR: Migrate Git HTTP traffic to Git HTTP rate limiter (!214087 - merged)

It's no longer needed in 18.8, as the migration is complete.

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