chore(spanner): lint fix

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITAbstractSpannerTest.java

@@ -35,7 +35,6 @@
 import com.google.common.base.Preconditions;
 import com.google.common.base.Stopwatch;
 import com.google.common.base.Strings;
-import com.google.common.collect.ImmutableList;
 import com.google.rpc.RetryInfo;
 import io.grpc.Metadata;
 import io.grpc.StatusRuntimeException;