Nextgen release: Don't build images that GSI don't need

These images are never used by GSI. Building them is a waste of resource
and a potential source of confusion, for they are packaged into the
*-img-*.zip of GSI. Skip building them.

Bug: 183068624
Test: "m dist" on GSI and check the build artifacts under OUT and DIST
  directories, and check the contents of *-img-*.zip
Change-Id: Ic14cfdd10ed26d975b369ce128e4b284378219ef