Remove the Element.prefix setter

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 74a3ef4a813d60f3362a5a7f32c53fe5e0a7350c..d896f74f7edc4ae9da7627ff080623113c3ecb3c 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -771,9 +771,6 @@ String UseCounter::deprecationMessage(Feature feature)
     case OverflowChangedEvent:
         return "The 'overflowchanged' event is deprecated and may be removed. Please do not use it.";
 
-    case ElementSetPrefix:
-        return "Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).";
-
     case SyncXHRWithCredentials:
         return "Setting 'XMLHttpRequest.withCredentials' for synchronous requests is deprecated.";