Fix the AuditUser by removing the redundant 'getCacheKey' method

The following compilation error was fixed:
  AuditUser.java:44: error: method getCacheKey() is already defined in class AuditUser
    public Object getCacheKey() {
                  ^

Change-Id: I89ea705f32907d7524cf93cddfa908217c51053b

src/main/java/com/googlesource/gerrit/plugins/auditsl4j/AuditUser.java

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 f57db23..b68670f 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);
   }