chromeos: Remove AshTestBase::SupportsMultipleDisplays

ash/mus/test/ash_test_impl_mus.cc

Index: ash/mus/test/ash_test_impl_mus.cc
diff --git a/ash/mus/test/ash_test_impl_mus.cc b/ash/mus/test/ash_test_impl_mus.cc
index e4e91867134c00951b4bfd8d2bc3143f0339dc00..71a44f4e7c415534eec90c800a761ca776513337 100644
--- a/ash/mus/test/ash_test_impl_mus.cc
+++ b/ash/mus/test/ash_test_impl_mus.cc
@@ -46,10 +46,6 @@ void AshTestImplMus::TearDown() {
   wm_test_base_->TearDown();
 }
 
-bool AshTestImplMus::SupportsMultipleDisplays() const {
-  return wm_test_base_->SupportsMultipleDisplays();
-}
-
 void AshTestImplMus::UpdateDisplay(const std::string& display_spec) {
   wm_test_base_->UpdateDisplay(display_spec);
 }