Revert "Add support for PEP 639 License-Expression."

This reverts commit 8bacee1.

Ref pypa/twine#1146

Author: jaraco

metadata.py

@@ -85,11 +85,10 @@ def discover(cls):
 
     @staticmethod
     def _discover_fields():
-        yield 'Metadata-Version', '2.4'
+        yield 'Metadata-Version', '2.3'
         yield 'Name', discovery.best_name()
         yield 'Version', discovery.version_from_vcs()
         yield 'Author-Email', discovery.author_from_vcs()
-        yield 'License-Expression', 'MIT'
         yield 'Summary', discovery.summary_from_github()
         yield 'Requires-Python', discovery.python_requires_supported()
         deps = list(discovery.combined_deps())