build: remove obsolete setup.py

setup.py was removed in favor of setup.cfg in #1626 and re-added
later in #1832 to work around a Dependabot issue #1828. This issue
seems to have been fixed upstream in dependabot/dependabot-core#5392.

Fixes #2089

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>