chore: Remove smoothScroll polyfill

[avinashbot]

Description

Safari 18 is out! That means all browsers (we support) now support smooth scroll. The only "browsers" that don't are basically JSDOM and friends, which will happily ignore behavior: "smooth" anyway. But there's still value in having the function because we want to turn it off when disableMotion(true) is called.

Related links, issue #, if available: n/a

How has this been tested?

Local tests, but I want to dry run it to be sure.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.61%. Comparing base (ba1929a) to head (e3a49a5). Report is 180 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tabs/scroll-utils.ts	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2823      +/-   ##
==========================================
- Coverage   96.19%   95.61%   -0.59%     
==========================================
  Files         761      760       -1     
  Lines       21413    21392      -21     
  Branches     6930     7321     +391     
==========================================
- Hits        20598    20453     -145     
- Misses        807      879      +72     
- Partials        8       60      +52     

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