javascript: make MaD sanitizer private

- in line with MaD sinks in other places
- placates the qldoc check

Author: yoff

javascript/ql/lib/semmle/javascript/security/dataflow/IncompleteHtmlAttributeSanitizationCustomizations.qll

@@ -101,7 +101,7 @@ module IncompleteHtmlAttributeSanitization {
     }
   }
 
-  class SanitizerFromModel extends Sanitizer {
+  private class SanitizerFromModel extends Sanitizer {
     SanitizerFromModel() { ModelOutput::barrierNode(this, "request-forgery") }
   }
 }