Change how chunk_copy() skips ghost not used from the source chunk

[backwardsEric]

Also use passed chunk argument rather than cave when resetting the ghost in delete_monster_idx().

Avoids an assertion failure copying over the monster groups when both chunks have a ghost.

This is cleaner than https://github.com/NickMcConnell/FAangband/pull/423 and possible now that delete_monster_idx() takes a chunk argument.

[NickMcConnell]

Thanks for following this up