deprecate DefiningIdentifier, it was not used in any query

Author: erik-krogh

javascript/ql/src/Declarations/Definitions.qll

@@ -1,7 +1,7 @@
 import javascript
 
-/** An identifier appearing in a defining position. */
-class DefiningIdentifier extends Identifier {
+/** DEPRECATED: An identifier appearing in a defining position. */
+deprecated class DefiningIdentifier extends Identifier {
   DefiningIdentifier() {
     this instanceof VarDecl or
     exists(Assignment assgn | this = assgn.getLhs()) or