Update Renovate config to remove commit message prefix

Renovate is currently configured to add SKIP UPDATE CHECK on initial commit/MR creation: https://repo1.dso.mil/big-bang/product/packages/loki/-/blob/main/renovate.json?ref_type=heads#L10

This should normally be removed once work starts on the Renovate ticket, but since having it in the first place it isn't actually required, we can just remove it altogether.