lint fixes.

Author: vverman

google-auth-library-java/cab-token-generator/java/com/google/auth/credentialaccessboundary/ClientSideCredentialAccessBoundaryFactory.java

@@ -53,7 +53,6 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Strings;
 import com.google.common.util.concurrent.AbstractFuture;
-import com.google.common.util.concurrent.FutureCallback;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
 import com.google.common.util.concurrent.ListenableFutureTask;
@@ -79,7 +78,6 @@
 import java.util.Date;
 import java.util.List;
 import java.util.concurrent.ExecutionException;
-import javax.annotation.Nullable;
 
 /**
  * A factory for generating downscoped access tokens using a client-side approach.