Clear old item groups when they are overridden. (#8753)

This fixes overridden items keeping their old groups in the group to
items mapping even after their groups have been changed in lua.
It also prevents a more widespread issue where overriding an item
will add its content ID *twice* to the mapping, resulting in odd
behaviour in features such as ABMs.

0 files changed, 0 insertions, 0 deletions