remove unsupported configure option

[TommyLau]

@CzBiX according to official website: http://www.infradead.org/ocserv/changelog.html

Version 0.9.0 (released 2015-01-20)

• Use a separate IPC and PID namespace in Linux systems for worker processes. That effectively puts each worker process in a separate container. This can be enabled at compile time using --enable-linux-namespaces.

I didn't see that configure option had been obsoleted. Correct me if I'm wrong.

[CzBiX]

Please search "unrecognized" in docker build logs or "namespace" in ocserv commit logs.

[TommyLau]

@CzBiX https://hub.docker.com/r/tommylau/ocserv/builds/b3vhw94pjwrjtphd5j82xqk/