Remove incorrect auto-generated header from error_message_target

Author: TheBlueMatt

fuzz/fuzz_targets/msg_error_message_target.rs

@@ -1,6 +1,3 @@
-// This file is auto-generated by gen_target.sh based on msg_target_template.txt
-// To modify it, modify msg_target_template.txt and run gen_target.sh instead.
-
 extern crate lightning;
 
 use lightning::ln::msgs;