Praefect: Remove clock drift readiness check

Currently the Praefect readiness RPC performs a clock drift check when invoked. This clock drift check calls the configured NTP service to validate the time against. If the NTP service call fails, the readiness check is considered failed. The clock drift check should not be included in the readiness check as it is more of a host problem. Also for deployments that depend on the readiness RPC to signal readiness, a failed response from the NTP service could inaccurately reflect the state of Praefect.

This change removes the clock drift check from the checks performed by the readiness RPC.