datastore/metadata: delete the entityGroupKind const

This is current un-used (and is currently incorrect), so remove it until
we need it.

Fixes #20

Change-Id: Iff1b887bc885a7b39e53f1bc9ab5f006fae27b5d
diff --git a/datastore/metadata.go b/datastore/metadata.go
index 3192a31..67995f9 100644
--- a/datastore/metadata.go
+++ b/datastore/metadata.go
@@ -8,10 +8,9 @@
 
 // Datastore kinds for the metadata entities.
 const (
-	namespaceKind   = "__namespace__"
-	kindKind        = "__kind__"
-	propertyKind    = "__property__"
-	entityGroupKind = "__entitygroup__"
+	namespaceKind = "__namespace__"
+	kindKind      = "__kind__"
+	propertyKind  = "__property__"
 )
 
 // Namespaces returns all the datastore namespaces.