Use omniauth nickname as the username for LDAP

Before there was a bug in omniauth-ldap which prevented samaccountname
showing up as a possible username for new LDAP users. Thanks to upstream
fixes, we no longer need to work around this bug.

CHANGELOG

@@ -2,6 +2,7 @@ v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion
   - Enabled GZip Compression for assets in example Nginx, make sure that Nginx is compiled with --with-http_gzip_static_module flag (this is default in Ubuntu)
   - Make user search case-insensitive (Christopher Arnold)
+  - Remove omniauth-ldap nickname bug workaround
 
 v 6.7.2
   - Fix upgrader script

lib/gitlab/ldap/user.rb

@@ -86,11 +86,7 @@ module Gitlab
         end
 
         def username
-          (auth.info.nickname || samaccountname).to_s.force_encoding("utf-8")
-        end
-
-        def samaccountname
-          (auth.extra[:raw_info][:samaccountname] || []).first
+          auth.info.nickname.to_s.force_encoding("utf-8")
         end
 
         def provider