TAGSTORE: Remove tag when its last RID is unset by TAGSTORE command

Resources use TAGSTORE to tell the Akonadi that tag has been removed on
the remote server. They simply unset RID of that tag (because tags have
per-resource RID). To emulate the situation, when all remote references
to a tag have been removed, we need to remove the tag in Akonadi server
and untag all items when the very last RID is removed by a resource.