Remove obsolete commands, remove "selected arena"

The notion of a "selected arena" disappears with this commit. All
commands that previously did not require an arena name in the case
of multiple arenas now do, i.e. it is no longer possible to "set"
an arena to be the target of commands. This should not be an issue,
however, since Setup Mode replaces all the commands that previously
were the main reason for the notion of a selected arena.