This method existed to allow LDAP users to take over existing GitLab
accounts if the part before the '@' of their LDAP email attribute
matched the username of an existing GitLab user. I propose to disable
this behavior in order to prevent unintended GitLab account takeovers.

After this change it is still possible to take over an existing GitLab
account with your LDAP credentials, as long as the GitLab account email
address matches the LDAP user email address.