Remove erroneous precondition for null surface test

When DisplayContent is mirroring a VirtualDisplay with no surface,
it is expected that querying the surface size returns null.

Test mistakenly included an override for this method, leading to
the test case asking SurfaceFlinger to mirror a null surface.

Bug: 200017844
Test: atest WmTests:DisplayContentTests#testVirtualDisplayContent_withoutSurface --iterations 20
Change-Id: Ibaef4b85b99b6a3829ef598e050393db1616fdee

services/tests/wmtests/src/com/android/server/wm/DisplayContentTests.java

@@ -2319,11 +2319,10 @@ public class DisplayContentTests extends WindowTestsBase {
         // mirror.
         setUpDefaultTaskDisplayAreaWindowToken();
 
-        // GIVEN SurfaceControl can successfully mirror the provided surface.
+        // GIVEN SurfaceControl does not mirror a null surface.
         Point surfaceSize = new Point(
                 mDefaultDisplay.getDefaultTaskDisplayArea().getBounds().width(),
                 mDefaultDisplay.getDefaultTaskDisplayArea().getBounds().height());
-        surfaceControlMirrors(surfaceSize);
 
         // GIVEN a new VirtualDisplay with an associated surface.
         final VirtualDisplay display = createVirtualDisplay(surfaceSize, null /* surface */);