Drop link to license file in pyproject.toml

[BrianJKoopman]

Description

A quick update before making our first release with the hatchling build backend.

Motivation and Context

This line resulted in PyPI trying to insert the full license text in the sidebar, which was ugly. Avoiding that.

How Has This Been Tested?

I noticed on a release candidate test release. Then I tested the fix on test.pypi.org.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Packaging change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.