deprecate a source class that wasn't used anywhere

Author: erik-krogh

go/ql/lib/semmle/go/security/Xss.qll

@@ -6,8 +6,11 @@ import go
 
 /** Provides classes and predicates shared between the XSS queries. */
 module SharedXss {
-  /** A data flow source for XSS vulnerabilities. */
-  abstract class Source extends DataFlow::Node { }
+  /**
+   * DEPRECATED: This class is not used.
+   * A data flow source for XSS vulnerabilities.
+   */
+  abstract deprecated class Source extends DataFlow::Node { }
 
   /** A data flow sink for XSS vulnerabilities. */
   abstract class Sink extends DataFlow::Node {