prevent updating deleted profile-fields with 3rd..

..party connections 

see here: https://www.woltlab.com/forum/index.php/Thread/227754-Facebook-Anmeldung-nicht-m%C3%B6glich/
Merge pull request #1765 from joshuaruesweg/patch-26

prevent updating deleted profile-fields with 3rd..
Merge branch '2.0'