always make user in external share manager explicit

[icewind1991]

Summary

relying on the session user is inconsistent as there might not be a logged in user, or the logged in user might not be the recipient of the share (reshares).

Also removes some dead code.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[OsipovAlexander11]

Hi @icewind1991, thanks for working on this — we're hitting exactly this issue in production on a ~3000-user Nextcloud deployment (currently on 33.0.4 and 34.0.2). occ files:scan crashes whenever it encounters a stale federated share left behind after remote-side deletion, forcing us to manually query oc_storages/oc_mounts for available != 1 and clean up by hand before every scan.

Given the milestone is set to Nextcloud 36, could this be backported to stable33 and stable34 once merged? It would help us (and likely other admins running federated shares on current stable branches) avoid this workaround until the next major release.