<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def server_shutdown(server_state):
 """ Shut down server subsystems. Remove PID file. """

    set_running( False )

    # stop API servers
    rpc_stop(server_state)
    api_stop(server_state)

    # stop atlas node
    server_atlas_shutdown(server_state)

    # stopping GC
    gc_stop()

    # clear PID file
    try:
        if os.path.exists(server_state['pid_file']):
            os.unlink(server_state['pid_file'])
    except:
        pass

    return True