Remove out-dated NatVis visualizer.

Author: michaelwoerister

src/etc/natvis/liballoc.natvis

@@ -185,12 +185,4 @@
       </ArrayItems>
     </Expand>
   </Type>
-
-  <Type Name="alloc::borrow::Cow&lt;*&gt;">
-    <DisplayString Condition="RUST$ENUM$DISR == 0x0">Borrowed({__0})</DisplayString>
-    <DisplayString Condition="RUST$ENUM$DISR == 0x1">Owned({__0})</DisplayString>
-    <Expand>
-      <Item Name="[value]" ExcludeView="simple">__0</Item>
-    </Expand>
-  </Type>
 </AutoVisualizer>