etcdserver: remove duplicated imports

Removed duplicated imports of package 'context' in server.go

etcdserver/server.go

@@ -60,7 +60,6 @@ import (
 	"github.com/coreos/go-semver/semver"
 	"github.com/coreos/pkg/capnslog"
 	"github.com/prometheus/client_golang/prometheus"
-	"golang.org/x/net/context"
 )
 
 const (