remove license reducancy in pyproject.toml

From https://packaging.python.org/en/latest/guides/writing-pyproject-toml/ : "If you are using a standard, well-known license, it is not necessary to use [the license] field. Instead, you should one of the classifiers starting with License ::"

pyproject.toml

@@ -8,7 +8,6 @@ version = "0.0.005"
 description = 'Invariant Causal Prediction for python'
 readme = "README.md"
 requires-python = '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
-license = {text = 'MIT'}
 authors = [{name = 'Jan Gleixner', email = '[email protected]'}]
 maintainers = [{name = 'Jan Gleixner', email = '[email protected]'}]
 keywords = ['statistics', 'casual-inference']