tools: patman: fix `pip install` with Python 3.12

tools: patman: fix `pip install` with Python 3.12

Installing patman with `cd ./tools/patman && pip install -e .` fails
with the error below.

As described in the error output below, the license line is not allowed
to be only defined in the setup.py. We remove the 'license' field
entirely, as the Python Packaging User Guide recommends using projects
classifiers instead[1] and we already set the GPL-2.0+ classifier.

> $ cd ./tools/patman && pip install -e .
> Obtaining file:///.../u-boot/tools/patman
>   Installing build dependencies ... done
>   Checking if build backend supports build_editable ... done
>   Getting requirements to build editable ... error
>   error: subprocess-exited-with-error
>
>   × Getting requirements to build editable did not run successfully.
>   │ exit code: 1
>   ╰─> [61 lines of output]
>       /tmp/pip-build-env-mqjvnmz8/overlay/lib/python3.12/site-packages/setuptools/config/_apply_pyprojecttoml.py:76:
>           _MissingDynamic: `license` defined outside of `pyproject.toml` is ignored.
>       !!
>
>       ********************************************************************************
>       The following seems to be defined outside of `pyproject.toml`:
>
>       `license = 'GPL-2.0+'`
>
>       According to the spec (see the link below), however, setuptools CANNOT
>       consider this value unless `license` is listed as `dynamic`.
>
>       https://packaging.python.org/en/latest/specifications/pyproject-toml/#declaring-project-metadata-the-project-table
>
>       To prevent this problem, you can list `license` under `dynamic` or alternatively
>       remove the `[project]` table from your file and rely entirely on other means of
>       configuration.
>       ********************************************************************************
>
>       !!

[1] https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Reviewed-by: Simon Glass <sjg@chromium.org>