Auto merge of #29595 - ohno418:remove-charset-related-from-multipart-form-data-encoding-algorithm, r=jdm

Remove unnecessary steps from "multipart/form-data encoding algorithm"

The specifications for the "multipart/form-data encoding algorithm" has changed [1], and the "_charset_" is not now handled here.

Remove no longer necessary steps from the "multipart/form-data encoding algorithm".

Similar to the "text/plain encoding algorithm" case fixed by f0818aa3 (Remove unnecessary steps from "text/plain encoding algorithm").

[1]: https://github.com/whatwg/html/pull/3645

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29593 (GitHub issue number if applicable)

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- [x]  These changes do not require tests because this patch doesn't expect actual behavior, just removing extra steps.

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