Do not load extra user backends when an upgrade is due

Whenever an upgrade is due, do not load extra user backends

Fixes https://github.com/owncloud/core/issues/10762

Steps:
1) Setup OC 6.0.4
2) Enable user_external (do not configure it)
3) Upgrade to this PR's branch

Before the fix: exception
After the fix: upgrade works

Please review/test @adamwill @icewind1991 @blizzz