Remove unused CurrentUser#cacheKey method

Change-Id: If7ccfd9a418a379c677a7150fd14a7fc2d497cd5

java/com/google/gerrit/server/CurrentUser.java

@@ -90,12 +90,6 @@ public abstract class CurrentUser {
    */
   public abstract GroupMembership getEffectiveGroups();
 
-  /**
-   * Returns a unique identifier for this user that is intended to be used as a cache key. Returned
-   * object should to implement {@code equals()} and {@code hashCode()} for effective caching.
-   */
-  public abstract Object getCacheKey();
-
   /** Unique name of the user on this server, if one has been assigned. */
   public Optional<String> getUserName() {
     return Optional.empty();