mcs tutorial: Simplify passive server initialization protocol example

Once https://github.com/seL4/seL4/pull/523 is merged, setting up a passive server becomes simpler and doesn't need a special init protocol anymore.  The mcs tutorial should take advantage of this when showing how to configure a passive server: https://github.com/seL4/sel4-tutorials/blob/master/tutorials/mcs/mcs.md#passive-servers-1