Remove a bunch of setup_parser plumbing from goal/task.

reflect.py relied on the old options system via setup_parser, so
it's been broken for a while, ever since options migration began.
Fixed that by making it read from the new options.

Testing Done:
Manually ran the doc generator and inspected the resulting docs.

CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1297/
Sync of https://github.com/pantsbuild/pants/commit/1f1a2b6a38f1842de090601b5e7e1782ce3d7088