Lint fix.

Author: vverman

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/RegionalAccessBoundaryManager.java

@@ -37,7 +37,6 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.util.concurrent.SettableFuture;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Level;