Remove break from hunk migration

Summary: This break statement causes ./bin/hunks migrate to only migrate one-hunk-at-a-time, which is unnecessary and slightly misleading. Instead, allow the script to migrate all legacy hunks to modern storage. In particular, this means that we can recommend that installs run this command sometime before D13222 is landed.

Test Plan: It's a pain to setup the data necessary to test this, but this is identical to the change that I made on our production install when I migrated our hunk storage.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13288