C#: Minor code quality improvements.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/internal/Overlay.qll

@@ -52,10 +52,10 @@ private class DiscardableEntity extends @locatable {
   }
 
   /** Holds if this element exists in the base variant. */
-  predicate existsInBase() { not isOverlay() and exists(this) }
+  predicate existsInBase() { not isOverlay() }
 
   /** Holds if this element exists in the overlay variant. */
-  predicate existsInOverlay() { isOverlay() and exists(this) }
+  predicate existsInOverlay() { isOverlay() }
 
   /** Gets a textual representation of this discardable element. */
   string toString() { none() }