C#: Deprecate unused SSA predicate getElement.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/dataflow/SSA.qll

@@ -422,11 +422,13 @@ module Ssa {
     }
 
     /**
+     * DEPRECATED.
+     *
      * Gets the syntax element associated with this SSA definition, if any.
      * This is either an expression, for example `x = 0`, a parameter, or a
      * callable. Phi nodes have no associated syntax element.
      */
-    Element getElement() { result = this.getControlFlowNode().getAstNode() }
+    deprecated Element getElement() { result = this.getControlFlowNode().getAstNode() }
 
     /** Gets the callable to which this SSA definition belongs. */
     final Callable getEnclosingCallable() {