Woops, remove leftover import.

Author: Michael Lehenbauer

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/ArrayTransformsTest.java

@@ -27,7 +27,6 @@
 import java.util.Map;
 import org.junit.After;
 import org.junit.Before;
-import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;