user-accounts: fix enterprise accounts deleting

Deleting enterprise accounts using act_user_manager_delete_user fails
with error: userdel failed, because enterprise accounts are added
using act_user_manager_cache_user and don't have entries in passwd
file. Thus enterprise accounts should be removed by
act_user_manager_uncache_user.

https://bugzilla.gnome.org/show_bug.cgi?id=727871