Only show currently linked items in /unlink autosuggest

Current behavior

/unlink shows all possible issues/work items as suggested options, however only linked items can actually be removed. This affects issues, and will affect work items once !173051 (merged) is merged.

A similar quick action, /unlabel, shows only applied labels.

Expected behavior

Only show linked items in the /unlink autosuggest.

Classification

bugux with severity3 as it can be rather cumbersome to find the correct item, significantly impeding the usage of this feature.