Deleted users appear in sysadmin user lists

[MrkGrgsn]

Sysadmin users that have been deleted still appear in the sysadmin list at /ckan-admin and in the output of a paster sysadmin list command in CKAN in 2.4.1 and 2.5.2 without any indication the user has been deleted.

I expect that deleted sysadmin users will not appear in either list or possibly appear with an indication they are deleted.

To reproduce:

1. Create a sysadmin user 'test_sysadmin'
2. Delete 'test_sysadmin' user
3. Open /ckan-admin - 'test_sysadmin' is listed
4. Run paster sysadmin list - 'test_sysadmin' is listed