Remove getCacheKey() from AuditUser

Adapt to the stable-3.3 and master branches, where the getCacheKey has
been removed from the CurrentUser.

Change-Id: I6edfc2f662ed97cd0b82bbc73257d5df09600a11
diff --git a/src/main/java/com/googlesource/gerrit/plugins/auditsl4j/AuditUser.java b/src/main/java/com/googlesource/gerrit/plugins/auditsl4j/AuditUser.java
index 2e6dd8a..0bb3280 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/auditsl4j/AuditUser.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/auditsl4j/AuditUser.java
@@ -27,11 +27,6 @@
   }
 
   @Override
-  public Object getCacheKey() {
-    return null;
-  }
-
-  @Override
   public Optional<String> getUserName() {
     return Optional.of(username);
   }