8277868: Use Comparable.compare() instead of surrogate code

Reviewed-by: rriggs, aivanov

src/java.base/share/classes/java/util/Calendar.java

@@ -3416,8 +3416,7 @@ public abstract class Calendar implements Serializable, Cloneable, Comparable<Ca
     }
 
     private int compareTo(long t) {
-        long thisTime = getMillisOf(this);
-        return (thisTime > t) ? 1 : (thisTime == t) ? 0 : -1;
+        return Long.compare(getMillisOf(this), t);
     }
 
     private static long getMillisOf(Calendar calendar) {