RelativeDateFormatter: Remove unnecessary @SuppressWarnings("boxing")

This suppression is not needed since we are ignoring warnings about
boxing in the compiler settings.

Having this suppression causes an "info" warning in Eclipse Oxygen.

Change-Id: Iff924a45f7d3d3e7f5a2c7a7fb95f82b1b21233d
(cherry picked from commit 56367dd838)

gerrit-gwtui-common/src/main/java/com/google/gerrit/client/RelativeDateFormatter.java

@@ -37,7 +37,6 @@ public class RelativeDateFormatter {
    * @return age of given {@link Date} compared to now formatted in the same relative format as
    *     returned by {@code git log --relative-date}
    */
-  @SuppressWarnings("boxing")
   public static String format(Date when) {
     long ageMillis = (new Date()).getTime() - when.getTime();