[cfe] Remove _Added

This is no longer needed since only fragments are created during outline
building.

Change-Id: Ie3fa3d0a3295020aa56d780a7cc87e4dd2409544
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391962
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther