FIX: Remove `username_lower` from admin serializer

FIX: Remove username_lower from admin serializer

The property is already computed on the front end and this ends up overwriting it.

diff --git a/app/serializers/admin_user_list_serializer.rb b/app/serializers/admin_user_list_serializer.rb
index e6695f5..eab8fe1 100644
--- a/app/serializers/admin_user_list_serializer.rb
+++ b/app/serializers/admin_user_list_serializer.rb
@@ -13,7 +13,6 @@ class AdminUserListSerializer < BasicUserSerializer
              :last_seen_age,
              :last_emailed_age,
              :created_at_age,
-             :username_lower,
              :trust_level,
              :manual_locked_trust_level,
              :flag_level,

GitHub sha: 549c5524