remove license classifier

[drammock]

I'm getting build errors like

********************************************************************************
Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: MIT License

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************

stemming from SetuptoolsDeprecationWarning: License classifiers are deprecated.

Since setup.py already includes a license = "MIT" field, it should be OK to simply remove this classifier. Building rtmixer from this fork/branch made the error go away.

[mgeier]

Thanks!