Revert "Remove getCacheKey() from AuditUser"

This reverts commit 4d6113617c7d2a241a55d48d186d26989894217d
because the corresponding Change-Id: If7ccfd9a on stable-3.3
has been reverted and released in Gerrit v3.3.1.

Change-Id: Ice7e83bfd4e24398a91dadf65a6c3ef963136603

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