Remove `discovery` group from CLI (#3830)
## Summary
Allows, e.g., `UV_SYSTEM_PYTHON=false uv pip install --python
.venv/bin/python`.
This was intended to work after fixing
https://github.com/astral-sh/uv/issues/3000, but I think I misdiagnosed
the scope when closing that issue, and the linked PR there only fixed
some _other_ problems around index URLs.
The only thing we really lose here is we no longer error when
`--break-system-packages` is provided without `--system`, but we can
enforce that elsewhere if we want.
Closes https://github.com/astral-sh/uv/issues/3829.
