LibCore: Remove .xht from common_extensions in MimeType text/html

[rcorsi]

Extension .xht was part of common_extensions for two MimeType array entries: application/xhtml+xml and text/html. Should only be part of one.

Duplicate .xht extension removed from text/html mime type and is now only part of application/xhtml+xml mime type.

Pull request as a follow-up to #4860.

As Andrew Kaster mentions here, an authoritative reference for .xht file is not easy to find.

Some mention of the extension and these MimeTypes, at the IANA mime type registry.

https://www.iana.org/assignments/media-types/application/xhtml+xml https://www.iana.org/assignments/media-types/text/html

This Mozilla guide does not include the .xht extension in this table, only makes reference to .xhtml, html, htm

https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/MIME_types/Common_types

Looking further online we have lots of discussion about html vs xhtml

https://www.w3.org/TR/2010/WD-html-markup-20101019/documents.html#mime-types https://www.sitepoint.com/community/t/html-or-xhtml/24939 https://www.w3schools.com/html/html_xhtml.asp https://stackoverflow.com/questions/1969290/what-is-the-difference-between-html-and-xhtml-extension-xhtml-is-a-markup-lan

I'm sure more can be found. I didn't look hard for these.

[rcorsi]

@ADKaster can you please have a look at this followup pull request when you have a few minutes? Should I rebase to latest, or is that not needed until there is a merge conflict?

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