Chore: Remove unused CSS for seamless panel styling

[kaixin-hc]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Fixes #2324

Overview of changes:
Remove the css related to card-seamless

Here is a recording of the behaviour of seamless panel after the changes, which is identical to the current behaviour
https://github.com/MarkBind/markbind/assets/59471320/eb53b809-2f2e-4de6-ad97-554839aed9d3

Anything you'd like to highlight/discuss:

Testing instructions:

markbind serve -d on markbind/docs, navigate to panel, see that behaviour is unchanged

Proposed commit message: (wrap lines at 72 characters)
Chore: Remove unused CSS for seamless panel

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

---

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (67ae4b1) 48.87% compared to head (f96a89e) 48.87%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2407   +/-   ##
=======================================
  Coverage   48.87%   48.87%           
=======================================
  Files         124      124           
  Lines        5238     5238           
  Branches     1109     1109           
=======================================
  Hits         2560     2560           
  Misses       2371     2371           
  Partials      307      307           

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[yucheng11122017]

LGTM! Thanks!