fix(frontend): Remove undefined style

[woxtu]

What

MkPreview 内の未定義なスタイルを削除します。

Why

ビルドするときに出る次の警告を解決したい：

Undefined style detected: $style.preview__content1 (in MkSample)

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

[kakkokari-gtyih]

🙏

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 20.15%. Comparing base (75b0315) to head (18b606c).
Report is 14 commits behind head on develop.

Files	Patch %	Lines
packages/frontend/src/components/MkPreview.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #14427       +/-   ##
============================================
- Coverage    39.90%   20.15%   -19.76%     
============================================
  Files         1547      725      -822     
  Lines       190875   100466    -90409     
  Branches      3569     1051     -2518     
============================================
- Hits         76166    20247    -55919     
+ Misses      114116    79676    -34440     
+ Partials       593      543       -50     

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