Finish migration from int64 durations to durationpb (#7147)

This is a cleanup PR finishing the migration from int64 durations to
protobuf `*durationpb.Duration` by removing all usage of the old int64
fields. In the previous PR
#7146 all fields were
switched to read from the protobuf durationpb fields.

Fixes #7097

ra/ra.go

@@ -1496,9 +1496,8 @@ func (ra *RegistrationAuthorityImpl) checkCertificatesPerFQDNSetLimit(ctx contex
 	}
 
 	prevIssuances, err := ra.SA.FQDNSetTimestampsForWindow(ctx, &sapb.CountFQDNSetsRequest{
-		Domains:  names,
-		WindowNS: limit.Window.Duration.Nanoseconds(),
-		Window:   durationpb.New(limit.Window.Duration),
+		Domains: names,
+		Window:  durationpb.New(limit.Window.Duration),
 	})
 	if err != nil {
 		return fmt.Errorf("checking duplicate certificate limit for %q: %s", names, err)