Remove license line in project section of pyproject.toml

[toshihikoyanase]

Motivation

In the metadata section of pypi, the license of optunahub is not shown fully.
I think this is because pyproject.toml imports the contents of the LICENSE file.
Instead, we can use 'classifiers' starting with License :: according to Python packaging user guide.

Optuna v3.0.0 used only the classifiers, but pypi showed the metadata as expected.

Description of the changes

• Removed the license in project

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.08%. Comparing base (e55a73d) to head (a7a75d6).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   81.95%   82.08%   +0.13%     
==========================================
  Files           6        6              
  Lines         133      134       +1     
==========================================
+ Hits          109      110       +1     
  Misses         24       24              

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