chromeos: Remove AshTestBase::SupportsMultipleDisplays

ash/mus/test/wm_test_base.cc

Index: ash/mus/test/wm_test_base.cc
diff --git a/ash/mus/test/wm_test_base.cc b/ash/mus/test/wm_test_base.cc
index 251d90bc5125419cccb83aa2da2c1f19c98bf09c..69210577d7c48f25a3997fcdb90b475f67957aad 100644
--- a/ash/mus/test/wm_test_base.cc
+++ b/ash/mus/test/wm_test_base.cc
@@ -68,10 +68,6 @@ WmTestBase::~WmTestBase() {
       << "You have overridden TearDown but never called WmTestBase::TearDown";
 }
 
-bool WmTestBase::SupportsMultipleDisplays() const {
-  return true;
-}
-
 void WmTestBase::UpdateDisplay(const std::string& display_spec) {
   test_helper_->UpdateDisplay(display_spec);
 }