R1:042c43d6d843

Remove a very old Herald garbage collection migration

Summary:
Ref T13048. This migration is from January 2012 and probably only impacted Facebook.

It references `HeraldRepetitionPolicyConfig`, which I'd like to change significantly. I initially just replaced the constant with a literal `0`, but I don't think there's any actual value in retaining this migration nowadays.

The cost of removing this migration is: if you installed Phabricator before January 2012 and haven't upgraded since then, you'll have…