Some command-line options are obsolete

Consider `-himem` for example. [`-himem` doesn’t do anything unless your system has less than 62 MiB of memory.](https://github.com/DescentDevelopers/Descent3/blob/496b2ed7c94a1fc08c6ed2831fc64abf486c7ab5/mem/mem.cpp#L413-L416) It seems pretty unlikely that a player would try to run a modern version of Descent 3 on a system with less than 62 MiB or memory, and if they do, then it’s probably fine if the game runs in low memory mode.

There’s probably at least a few different command-line options that are no longer relevant, and it would be good to remove them.