dropping generation of old devhelp format in gtk-doc

[Stefan Kost <ensonic hora-obscura de>]

Hi,

In 2005 we created a new format for devhelp index files that contains more
details. It is supported since devhelp-0.11 (18.Dec.2005). The next release of
gtk-doc will not generate the old devhelp files anymore. This speeds up the
builds a bit and saves some bytes on your hard-dives :)

If I am overlooking something here, please let me know.

Thanks,
Stefan