Auto merge of #29584 - ohno418:fix-text-plain-encoding-algorithm, r=jdm

Remove unnecessary steps from "text/plain encoding algorithm"

For "text/plain encoding algorithm", the specification [1] has changed [2] and "_charset_" is not now handled here. It's supposed to be handled in "construct the form data set" algorithm, and we've already implemented that [3]. So we now have extra steps for "text/plain encoding" algorithm.

Remove no longer necessary steps from text/plain encoding algorithm so that it meets the specification.

[1]: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#text/plain-encoding-algorithm
[2]: https://github.com/whatwg/html/pull/3645
[3]: https://github.com/servo/servo/pull/25217

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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 #25221 (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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