Changes in pipe_tasks will take care of making existing tasks use this
new task's outputs instead of the old visit summaries, so most of the
changes here are about redefining subsets to be compatible with the
new overall flow.

Many of the DECam changes are actually removals of Gen2 config options
that now do nothing (but previously did the equivalent of Gen3
connection changes).

Now that we're using the final Piff PSF measurements to drive coadd
selection and other visit-summary-based things, we can switch back to
PSFEx for speed and robustness for our first PSF model.

This pipeline is soon to be deprecated anyway, but since we're still
nominally maintaining it it's useful to drop these tasks that don't
serve a purpose there: they're all things that really only need to be
run once, before the fakes are inserted, and rerunning them after the
fakes have been inserted either doesn't change them or adds circular
measurements (e.g. PSF summary stats on fakes inserted with the PSF
model we're trying to summaries) that we don't.

This hasn't been the actual pipeline run for DP0.2 for a long time,
and (unlike HSC's RC@) the ImSim test-med-1 pipeline is already just
as suitable for large-scale productions (it differs only in which
analysis plots/metrics are run).