engine: fix a bug in engine being unreachable
engine: fix a bug in engine being unreachable

When gRPC turned on, TestScheduleMachineOf fails sometimes, as the
engine becomes unreachable with the following error messages:

====
transport: http2Client. notifyError got notified that the client transport was broken EOF.
ERROR registrymux.go:166: Retry to connect to new engine: dial tcp 172.18.1.1:50059: getsockopt: connection refused
ERROR registrymux.go:166: Retry to connect to new engine: dial tcp 172.18.1.1:50059: getsockopt: connection refused
ERROR registrymux.go:166: Retry to connect to new engine: dial tcp 172.18.1.1:50059: getsockopt: connection refused
====

This must have been a regression from commit ecb121a ("registry/rpc: use
simpleBalancer instead of ClientConn.State()"). Remove the additional
checking with IsRegistryReady, in order to avoid the occasional case of
engine being unreachable.

Fixes https://github.com/coreos/fleet/issues/1712

giantswarm/fleet,endocode/fleet,dongsupark/fleet,jonboulle/fleet,dongsupark/fleet,endocode/fleet,giantswarm/fleet,endocode/fleet,coreos/fleet,jonboulle/fleet,jonboulle/fleet,coreos/fleet,dongsupark/fleet,giantswarm/fleet,coreos/fleet