MimeTypes: Remove unused html subtype symlinks

These html subtypes now fallback to `text-html`, according
to shared-mime-info 2.4, so these symlinks to `text-html`
should no longer be needed.

Tested in Thunar, Caja, and Nemo with no change in the
displaying of proper icons for several different html
subtypes/extensions (mhtml, xht, etc).

Author: newhoa