Removal of link attribute in guide.dtd

Hi guys,

I will be removing the link attribute within the guide.dtd soon so that
newly committed changes (and documents) cannot use the "<guide link=.../>"
stuff anymore. See also bug #379883 [1].

Wkr,
	Sven Vermeulen

[1] https://bugs.gentoo.org/show_bug.cgi?id=379883