Revert "Actually use the 'user_filter' configuration option"

This reverts commit e9d4587f, 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))`.

config/initializers/devise.rb

@@ -223,7 +223,6 @@ Devise.setup do |config|
       method:   Gitlab.config.ldap['method'],
       bind_dn:  Gitlab.config.ldap['bind_dn'],
       password: Gitlab.config.ldap['password'],
-      filter:   Gitlab.config.ldap['user_filter'],
       name_proc: email_stripping_proc
   end
 
@@ -245,4 +244,4 @@ Devise.setup do |config|
 
     config.omniauth provider['name'].to_sym, *provider_arguments
   end
-end
\ No newline at end of file
+end