Remove deprecated artifact features from API document

[c-bata]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

To prevent users continue to use deprecated features.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4148415) 69.39% compared to head (4235419) 69.39%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #789   +/-   ##
=======================================
  Coverage   69.39%   69.39%           
=======================================
  Files          35       35           
  Lines        2369     2369           
=======================================
  Hits         1644     1644           
  Misses        725      725           

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