Fix XMPPAuthenticator to support non-optimized message flow

While RFC6120 allows and encourages sending data with the success
message it is also legal to send the same data as a challenge and
await an empty response. This rework honors that fact.

src/XMPPPLAINAuth.m

@@ -44,7 +44,7 @@
 				     password: password] autorelease];
 }
 
-- (OFDataArray*)clientFirstMessage
+- (OFDataArray*)initialMessage
 {
 	OFDataArray *message = [OFDataArray dataArrayWithItemSize: 1];
 
@@ -68,17 +68,4 @@
 
 	return message;
 }
-
-- (OFDataArray*)calculateResponseWithChallenge: (OFDataArray*)challenge
-{
-	@throw [XMPPAuthFailedException newWithClass: isa
-					  connection: nil
-					      reason: @"Received a challenge "
-						      @"during PLAIN auth"];
-}
-
-- (void)parseServerFinalMessage: (OFDataArray*)message
-{
-	return;
-}
 @end