ratelimit: Replace RateLimitConfig with rate.Limit
Created by: ryanslade
We no longer need to return a RateLimitConfig
type with extra metadata. All we need is the actual
rate limit. This data was needed in the past for some error messages we passed back when saving a
config with rate limiting but that was removed a while back.
Test plan
Only removal of code that was no longer used. All tests still pass.