ff-iii used to have LDAP but we removed it. in the database, 'sso'
is still true. the migration here will reset it back to false.
for future situations like these, we sync the sso flag on app update itself.
this ensures correct behavior when yet another update add back sso support.
in ff-iii case, a future update is bringing in proxyAuth based sso!
we don't store the 'sso' bit in backupdb, so user choice of sso is
lost if restore changes sso addons.
