Remove loaded_via_alias

Remove loaded_via_alias

This effectively reverts 707b43736d838436777f53b230e64054ecb244c5.

I have checked that this field is not used anywhere, including in any
relevant serialised output eg via the command listener.  As for game
save compatibility, we can simply ignore it on read (and, even go
backwards since it's Option).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>