Gardening.md: remove Go2 label from untriaged issues link

Change-Id: Ie56c38f30c9554b455933a859765fc33700f87c8
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/644715
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: David Chase <drchase@google.com>