Revert "Actually use the 'user_filter' configuration option"

This reverts commit e9d4587ff11c8510f01dfa184414f73d75b4550b, 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](https://gitlab.com/gitlab-org/gitlab-ce/blob/982d4d51e8110bec280eb00db0fb756b062103d9/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](https://gitlab.com/gitlab-org/omniauth-ldap/blob/76d77543dec0c585bb4e974262f43675f9810953/lib/omniauth/strategies/ldap.rb#L55)

Example omniauth-ldap user filter:
`(&(uid=%{username})(memberOf=cn=foo,dc=bar))`.
Merge branch 'fix_ldap_user_filter' into '6-9-stable'

Fix LDAP user filter