Delete unused prepared statement code.

Prepared statements are not supported; see MySqlCommand.Prepare for the
rationale.

Author: bgrainger

src/MySqlConnector/Serialization/CloseStatementPayload.cs

@@ -7,9 +7,6 @@ internal enum CommandKind
 		Query = 3,
 		Ping = 14,
 		ChangeUser = 17,
-		PrepareStatement = 22,
-		ExecuteStatement = 23,
-		CloseStatement = 25,
 		ResetConnection = 31,
 	}
 }