meson: remove pointless gtk-doc check
There's no point explicitly checking for gtk-doc, for two reasons:
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In cross-compiled environments this looks for a target gtk-doc, not a native gtk-doc, so it should set native:true.
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The gnome.gtkdoc() function checks for gtk-doc itself.
This isn't relevant for gtksourceview-5 onwards as gtkdoc has been replaced with gidocgen.