The `dev` output is empty anyway.
The problem is that it interacts badly with other parts of python and
stdenv infrastructure. In particular, before this patch it installs
code into `out` output (with only generated `nix-support` in `dev`),
but `makePythonPath` then uses `propagatedBuildInputs` to generate
`PYTHONPATH` while stdenv selects `dev` outputs for
`propagatedBuiltInputs`. This results in `makePythonPath` linking to
the empty `dev` output in `PYTHONPATH`.
This reverts a piece of commit 28299f669a.
