Clean up unused using alias in cast_remoting_connector_unittest.cc

This CL cleans up unused using aliases of MirrorServiceRemoter{Ptr,
Request} and MirrorServiceRemotingSource{Ptr, Request} in
cast_remoting_connector_unittest.cc since old Mojo types of them
was already converted.

Bug: 955171
Change-Id: I0f7fc2717418b9f752728435541d8f466593e625
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1882261Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#710131}

chrome/browser/media/cast_remoting_connector_unittest.cc

@@ -38,10 +38,6 @@ using media::mojom::RemotingSourcePtr;
 using media::mojom::RemotingSourceRequest;
 using media::mojom::RemotingStartFailReason;
 using media::mojom::RemotingStopReason;
-using media::mojom::MirrorServiceRemoterPtr;
-using media::mojom::MirrorServiceRemoterRequest;
-using media::mojom::MirrorServiceRemotingSourcePtr;
-using media::mojom::MirrorServiceRemotingSourceRequest;
 
 using ::testing::_;
 using ::testing::AtLeast;