Remove the (already disabled) OverflowChangedEvent runtime flag

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index 5c9ebde44d8b8ebdfcf0b7fe1248fc75e93355f6..e89ae9f000be7fbb390caaf961a2d22e4e1a357d 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -805,9 +805,6 @@ String UseCounter::deprecationMessage(Feature feature)
     case RangeDetach:
         return "'Range.detach' is now a no-op, as per DOM (http://dom.spec.whatwg.org/#dom-range-detach).";
 
-    case OverflowChangedEvent:
-        return "The 'overflowchanged' event is deprecated and may be removed. Please do not use it.";
-
     case SyncXHRWithCredentials:
         return "Setting 'XMLHttpRequest.withCredentials' for synchronous requests is deprecated.";