Revert "Don't let trains load trains"

Revert "Don't let trains load trains"

This reverts commit b1a0ff2fbd87d3c332de1e0a41b3ddea133c77b1.

Land units with capability heavy can't be loaded on anything, and land
units carrying land units can't be loaded regardless of capabilities.
Commit b1a0ff2fbd8 additionally outlawed loading of trains on land
units, but not on ships.  Bad idea, because it complicates matters for
no good reason.  Revert it.

Doesn't affect the stock game, because its only train is heavy.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>