mw/etcd: revert 'add fallthrough'

This removes 'fallthrough' for *etcd* which is not needed. This was
added in 00f5c7797 but is totally not needed and creates backwards
incompat behavior even.

Thanks to @johnbelamaric for pointing this out in #925.

middleware/etcd/multi_test.go

@@ -15,7 +15,6 @@ import (
 func TestMultiLookup(t *testing.T) {
 	etc := newEtcdMiddleware()
 	etc.Zones = []string{"skydns.test.", "miek.nl."}
-	etc.Fallthrough = true
 	etc.Next = test.ErrorHandler()
 
 	for _, serv := range servicesMulti {