[v10.2.x] Alerting: Fix deleting rules in a folder with matching UID in another organization

[papagian]

Backport 6d4625a from #78258

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What is this feature?

The deleteFolderAlertRules() function that is declared inside dashboard service has become obsolete since #67259 (alert rules get deleted here using DeleteInFolder()) but it was still got called.
Unfortunately, this function doesn't take into account the org_id. As a result:

• trying to delete a folder without alerts with matching folder UID in another organization (with alerts) wrongly fails with folder cannot be deleted: folder contains alert rules
• deleting a folder using forceDeleteRules=true would wrongly delete alert rules with matching folder (namespace_uid) UID in another organization (with alerts)

Why do we need this feature?

It fixes the above issues by removing the obsolete call.

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.