Fix bogus message when deity attempts to march foreign land unit

Fix bogus message when deity attempts to march foreign land unit

Much of the code assumes that only the land unit's owner can march it.
The assumption is correct, because lnd_mar() leaves foreign land units
behind with a bogus "was disbanded at" message (suppressed for country

It would be nice to let deities march foreign land units, but the
assumption is not trivial to remove.  For now, just avoid the bogus
message.

Historical note: it looks like deities used to be able to march
foreign land units just fine until Empire 2 factored common code out
of navigate, sail and autonav, and updated march to match navigate.
Likewise, it looks like they could board with foreign land units until
Empire 2 factored out common ground combat code.  Commands attack and
assault have always rejected foreign land units, even for deities.