[schema] drop meta property, deprecated since yams 0.25 (though warning was missing)

--HG--
branch : stable

schema.py

@@ -45,7 +45,7 @@ class EmailPart(EntityType):
         }
 
     content  = String(fulltextindexed=True)
-    content_format = String(required=True, meta=True, maxsize=50)
+    content_format = String(required=True, maxsize=50)
     ordernum = Int(required=True)
     alternative = SubjectRelation('EmailPart', symmetric=True)