Fix mentions not being deleted efficiently

[ClearlyClaire]

To further validate the speed gains from our recent optimizations, I tried running time ./bin/tootctl domains purge mastodon.social -c 1 --verbose with the old and new code.

The old code raised:

Removed 12223 accounts
Removed 66 custom emojis

real 103m18.756s
user 83m44.863s
sys 1m20.069s

while I had to stop the new one because it was still stuck at the third account after more than 40 minutes.

It turns out, as a regression from the recent optimizations, mentions were left untouched until account.destroy, which would then delete them individually, and executing queries to find and delete associated notifications, resulting in a massive slowdown.

With this change, running tootctl domains purge raises:

Removed 12223 accounts
Removed 66 custom emojis

real 22m52.500s
user 12m9.713s
sys 0m28.972s