Drop deprecated license classifier (astral-sh#24808)

## Summary

Per
astral-sh#19599 (comment),
maturin now supports PEP 639 fully. Consequently we should drop the
license classifier, as it's legally ambiguous.

See also astral-sh/uv#19130 for where we're
doing the same for uv.

## Test Plan

NFC.

Signed-off-by: William Woodruff <william@astral.sh>

Author: woodruffw

pyproject.toml

@@ -23,7 +23,6 @@ classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: MIT License",  # for compatibility with tooling such as pip-licenses
     "Operating System :: OS Independent",
     "Programming Language :: Python",
     "Programming Language :: Python :: 3.7",