[g2] remove <g2 targets from json
[g2] remove <g2 targets from json

When g1.5 reads g2 manifests, it picks a target that matches based on map
iteration order, which is poorly defined behavior.

Change-Id: I0378aaba3e5d818dc9d2ee3fe0138cf7de1186fc

theupdateframework/go-tuf