Bug 1069208 — Remove old 0.13 migration code.

I double checked, the 0.13 was released on November 6th and the 0.13.1 on November 13th then 0.13.2 on November 26th. So we can say that 0.13 branch was in production before November 26th and as we keep data for 1 month in the database all the migration would have been done before December 26th.

Here is the PR that remove the migration code.

https://bugzilla.mozilla.org/show_bug.cgi?id=1069208