[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
diff --git a/repo.go b/repo.go
index 6b507f1..03da902 100644
--- a/repo.go
+++ b/repo.go
@@ -526,6 +526,8 @@
 			meta.Custom = t.Custom
 		}
 
+		// G2 -> we no longer desire any readers to ever observe non-prefix targets.
+		delete(t.Targets, "/"+path)
 		t.Targets[path] = meta
 		return nil
 	}); err != nil {
@@ -571,6 +573,8 @@
 				continue
 			}
 			removed = true
+			// G2 -> we no longer desire any readers to ever observe non-prefix targets.
+			delete(t.Targets, "/"+path)
 			delete(t.Targets, path)
 		}
 		if !removed {