Drop the GCM RECEIPT message type (unused).

service/src/main/java/org/whispersystems/textsecuregcm/push/GCMSender.java

@@ -70,7 +70,6 @@ public class GCMSender implements Managed {
     String key;
 
     switch (message.getType()) {
-      case RECEIPT:      key = "receipt";      break;
       case NOTIFICATION: key = "notification"; break;
       case CHALLENGE:    key = "challenge";    break;
       default:           throw new AssertionError();

service/src/main/java/org/whispersystems/textsecuregcm/push/GcmMessage.java

@@ -7,7 +7,7 @@ import java.util.Optional;
 public class GcmMessage {
 
   public enum Type {
-    RECEIPT, NOTIFICATION, CHALLENGE
+    NOTIFICATION, CHALLENGE
   }
 
   private final String           gcmId;