Remove redundant licence definition

[DimitriPapadopoulos]

Drop the licence key, keep the licence classifier.

From the Python Packaging User Guide:

license

This can take two forms. You can put your license in a file, typically LICENSE or LICENSE.txt, and link that file here:

[project]
license = {file = "LICENSE"}

or you can write the name of the license:

[project]
license = {text = "MIT License"}

If you are using a standard, well-known license, it is not necessary to use this field. Instead, you should use one of the classifiers starting with License ::.