Remove adjourn from the list of the XBoard protocol features.

I'm doing this because the adjourn support for XBoard engines is broken in
Knights for the following reasons:
1. Saving the game isn't done properly; we were using the "save" command,
which is not an XBoard command; it's a custom command of gnuchess and
crafty, which, by the way, behaves different in each one.
2. There isn't any proper way to load the game. Not even typing
"load /file/path" in the console (that doesn't work because the board in
the GUI won't be updated).

We could implement adjourn support properly later using the "setboard" command,
see: