plugin/health: cleanups (#2811)

Small, trivial cleanup: got triggered because I saw a comment on how
health plugins polls other plugins which isn't true.

* Remove useless newHealth function
* healthParse -> parse
* Remove useless constants

Net deletion of code.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/health/overloaded.go

@@ -18,6 +18,7 @@ func (h *health) overloaded() {
 	}
 	url := "http://" + h.Addr
 	tick := time.NewTicker(1 * time.Second)
+	defer tick.Stop()
 
 	for {
 		select {
@@ -32,7 +33,6 @@ func (h *health) overloaded() {
 			HealthDuration.Observe(time.Since(start).Seconds())
 
 		case <-h.stop:
-			tick.Stop()
 			return
 		}
 	}