www/gtkhtml3: markd deprecated

From https://wiki.gnome.org/RoadMap/GtkHtml (2012-09-19):

• GtkHtml is being phased out in favor of WebKit/GTK+.
• Using GtkHtml in newly-written code is strongly discouraged.
• GtkHtml 4.6.x will likely be the final release series.

Upstream called this one dead 8 years ago. Time make it so.

PR: 252618
Submitted by: daniel.engberg.lists@pyret.net