This removes the pref to re-enable the legacy wizard, and all of the things that used
to read from it.
It also disables the ability to use the legacy wizard from about:welcome with the
aboutWelcomeBehavior feature.
It also removes the closeMigrationWizard utility from BrowserTestUtils, since that
effectively became a thin-wrapper around BrowserTestUtils.removeTab.
Finally, this patch also removes some documentation about the legacy migration wizard,
and tests that specifically check the legacy dialog behaviour.
Differential Revision: https://phabricator.services.mozilla.com/D190716
3c473874e4
