chore(spanner): lint fix

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/AsyncTransactionManagerImplTest.java

@@ -16,18 +16,14 @@
 
 package com.google.cloud.spanner;
 
-import static org.junit.Assert.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
-import static org.mockito.Mockito.clearInvocations;
-import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import com.google.api.core.ApiFutures;
 import com.google.cloud.Timestamp;
-import com.google.protobuf.ByteString;
 import io.opentelemetry.api.trace.Span;
 import io.opentelemetry.context.Scope;
 import org.junit.Test;