Fix failed AE migrations

Intent

Migration script was failing on orphaned active effects, which pointed to an item that no longer exists. We just delete these effects instead of trying to do anything with them.

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• This is meant for a hotfix
• This is meant for the next release (see milestone)
• This needs more reviewers than normal; there may be controversy or high complexity
• This intentionally introduces regressions that will be addressed later
• There is/will be documentation changes on the wiki
• Please do not send commits here without coordinating closely with the owner
• This is a Build System change

Related Issues

• #907 (closed)

Steps to Test

1. Download the world in #907 (closed)
2. Checkout this branch.
3. Migrate that world (note, this took about 20 minutes for me).
4. Migration completes with no errors

Additional Notes

Not sure why there were orphaned effects in the first place but this takes care of them.