Remove legacy rate limits code

[beautifulentropy]

• Remove nearly all of the countRegistrationsByIP, except the supporting field InitialIP in core.Registration. Default this field to "0.0.0.0" in the WFE and the SA (ratelimit: Remove legacy registrations per IP implementation #7760)
• Remove InitialIP from core.Registration
• Remove all code referencing certificatesPerName table
• Remove all code referencing newOrdersRL table
• Drop certificatesPerName and newOrdersRL tables in db-next schema
• Deprecate DisableLegacyLimitWrites feature flag and delete all code it conditions
• Deprecate UseKvLimitsForNewOrder flag and delete all code it conditions
• Remove if wfe.limiter == nil && wfe.txnBuilder == nil in checkNewAccountLimits and checkNewOrderLimits WFE methods.
• Remove if ra.limiter == nil && ra.txnBuilder == nil in countFailedValidations() and countCertificateIssued() RA methods.