enhance(frontend): drop pref migration

[syuilo]

What

2025.4.0 以前の設定情報の移行処理を削除

Why

一年以上経過し概ね役割を果たした

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.61%. Comparing base (78435dc) to head (3b2a3f2).
⚠️ Report is 285 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #17379      +/-   ##
===========================================
+ Coverage    13.97%   14.61%   +0.64%     
===========================================
  Files          237      245       +8     
  Lines        11273    12148     +875     
  Branches      3728     4126     +398     
===========================================
+ Hits          1575     1776     +201     
- Misses        7578     8124     +546     
- Partials      2120     2248     +128     

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