Remove ExternalStorageProvider support from LockableDatabase

[cketti]

In the distant past K-9 Mail allowed the user to store the message database on external storage. We've stopped allowing this a very long time ago. But we've kept the code to move the message database from external storage to internal storage when users update from old K-9 Mail versions.

This change removes that migration code. That means when updating from very old K-9 Mail versions, accounts that store the message database on external storage will start over with an empty message database. I doubt this will be a problem in practice. It's not an issue at all for Thunderbird, since it never supported storing the message database on external storage.

Removing this code will eliminate one possible cause for #8516.