quarantine controller: drop specialized classes, check at runtime instead

quarantine controller: drop specialized classes, check at runtime instead

By making use of the optional chaining operator, we don't need to
override the 'onSelectMail' in both the attachment quarantine and the
spam quarantine. Instead, we opportunistically call lookup(id)?.setID
in the common handler for both, the attachments and the spaminfo
references.

As that was the single change from the parent class we can now drop
the specialized controller classes.

While at it, remove the stray 'xtype' property of the quarantine
controller, which make only sense for component derived classes.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [T: fix typos and reword message a bit]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>