Remove redundant HTMLToken::IsAll8BitData() & LiteralBuffer::AsString8()

LiteralBuffer: :AsString() already properly handles both all-8- and 16-bit strings, there's no need to do the same dance on the call site.
Change-Id: I22400527f86de627e48483bb264d2a34e387e3e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6419404
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1441115}