Remove WaitHandleExtensions, as it is implemented in CoreFX (#7431)

Author: kouvel

src/mscorlib/model.xml

@@ -8137,10 +8137,6 @@
       <Member Name="WaitOne(System.TimeSpan,System.Boolean)" />
       <Member MemberType="Property" Name="SafeWaitHandle" />
     </Type>
-   <Type Name="System.Threading.WaitHandleExtensions">
-      <Member Name="GetSafeWaitHandle(System.Threading.WaitHandle)" />
-      <Member Name="SetSafeWaitHandle(System.Threading.WaitHandle,Microsoft.Win32.SafeHandles.SafeWaitHandle)" />
-    </Type>
     <Type Name="System.Threading.WaitHandleCannotBeOpenedException">
       <Member Name="#ctor" />
       <Member Name="#ctor(System.String)" />

src/mscorlib/mscorlib.shared.sources.props

@@ -732,7 +732,7 @@
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\Timer.cs" />
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\Volatile.cs" />
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\WaitHandle.cs" />
-    <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\WaitHandleExtensions.cs" />
+    <ThreadingSources Condition="'$(FeatureCoreClr)'!='true'" Include="$(BclSourcesRoot)\System\Threading\WaitHandleExtensions.cs" />
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\WaitHandleCannotBeOpenedException.cs" />
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\ApartmentState.cs" />
     <ThreadingSources Include="$(BclSourcesRoot)\System\Threading\SpinLock.cs" />

src/mscorlib/ref/mscorlib.cs

@@ -13459,13 +13459,6 @@ public WaitHandleCannotBeOpenedException(string message) { }
         public WaitHandleCannotBeOpenedException(string message, System.Exception innerException) { }
         protected WaitHandleCannotBeOpenedException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) : base(info, context) { }
     }
-    public static partial class WaitHandleExtensions
-    {
-        [System.Security.SecurityCriticalAttribute]
-        public static Microsoft.Win32.SafeHandles.SafeWaitHandle GetSafeWaitHandle(this System.Threading.WaitHandle waitHandle) { throw null; }
-        [System.Security.SecurityCriticalAttribute]
-        public static void SetSafeWaitHandle(this System.Threading.WaitHandle waitHandle, Microsoft.Win32.SafeHandles.SafeWaitHandle value) { }
-    }
     [System.Runtime.InteropServices.ComVisibleAttribute(true)]
     public delegate void WaitOrTimerCallback(object state, bool timedOut);
 }