In the upgrade documents:

http://doc.ez.no/eZ-Publish/Upgrading/Upgrading-to-5.0/Upgrading-from-4.7-to-5.0, and
http://doc.ez.no/eZ-Publish/Upgrading/Direct-upgrading/Direct-upgrading-to-5.1-from-4.7/Direct-upgrading-from-4.7-to-5.1.

It's refered the optional step do remove admin2 from design settings. However by disabling this the extensions that use admin2 design will not work. (i.e. ezcomments, ezfind, etc...)