Remove inode/directory from MailCommon::FolderCollectionMonitor

Don't watch collections in KMail that KMail is not really interested in.
This should improve performance a bit and fix a problem, that removing a
completely unrelated resource (like Google Calendar) was totally messing
up KMail's folder view (usually leading to crash).

I assume this was added as a workaround for a bug in Monitor, that collections
trees where the root collection did not match the mimetype filter were ignored.
This has been fixed some time ago, so this is not needed anymore.

REVIEW: 109259