Remove use of Timeout.timeout from LDAP adapter

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We use Timeout.timeout in lib/gitlab/ldap/adapter.rb, but this function is broken in Ruby and does not play well with threads:

https://github.com/rails/rails/issues/22031#issuecomment-150378180

This may be causing issues in a customer's installation, but we don't have hard proof yet: https://gitlab.zendesk.com/agent/tickets/56118