This reverts commit e9d4587ff1, which is
incompatible with GitLab's built in LDAP user filter: a GitLab LDAP
filter is [added on top of the other filters used to find the
user](982d4d51e8/lib/gitlab/ldap/adapter.rb (L61))
Example GitLab LDAP filter: `(memberOf=cn=foo,dc=bar)`.
In contrast, an omniauth-ldap filter [replaces the 'normal'
filters](76d77543de/lib/omniauth/strategies/ldap.rb (L55))
Example omniauth-ldap user filter:
`(&(uid=%{username})(memberOf=cn=foo,dc=bar))`.