Use descriptive key for array of in a Special:EditWatchlist line

Use descriptive key for array of  in a Special:EditWatchlist line

WatchlistEditorBuildRemoveLine allows subscribers to modify $tools array.
However, it's hard to know exactly which link is which: $tools is just an array
of HTML links.
By adding a descriptive key, it'd be easier to unset or modify a specific link.

AFAICT, there currently are no subscribers to this hook in WMF extensions (Flow
will be coming up with one soon), so this change should be safe.

Change-Id: Id3f69db1aab7d3e3761318d7f8588ae63a0bc6a6