Remove Unicode newline characters U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR preprocessing from the Markdown preview.

This behavior was introduced in #63936 to address #63749, but it breaks the document in multiple ways:

• double-click word selection does not work as expected,
• screen readers may announce the text incorrectly,
• line wrapping and hyphenation does not work correctly,
• markdown-it plugins that rely on \b or \s may work incorrectly,
• even linkify fails:

https://foo.bar/baz<LS>Lorem ipsum
(Expected link: https://foo.bar/baz)
(Received link: https://foo.bar/bazLorem)

• and so on.

Since then it was fixed at the markdown-it side, and #63936 is no longer needed:

• markdown-it/markdown-it@d9cb3cc
• markdown-it/markdown-it@faecae0
• https://github.com/markdown-it/markdown-it/blob/9.1.0/lib/rules_core/normalize.js

Fixes #166015