Disable $wgUseTidy: not working and we're not interested in it

According to bawolff, this misleading HTML comment in all pages usually means
the binary is not found: "Tidy found serious XHTML errors".
Probably due to what explained in [[mw:$wgTidyInternal]].
According to Nikerabbit, we're not interested in tidy.

Change-Id: Ib7e433297fde220b1c2605cd3ef90db8f53bf5cb
Merge "Disable $wgUseTidy: not working and we're not interested in it"