doc/sphinx: No need for doxygen custom target

Unfortunate left over which was not removed once we had the script in place to regenerate the documentation. A side-effect was the fact that we still ran the script even if 'docs' target was called, effectively nullifying the fact that 'docs' target was ran at all. For instance, 'install' would've been executing the script even if 'docs' target was called before.

This removes the doxygen_target entirely and the depends of sphinx target on it, makes building and installing docs a bit faster.

Signed-off-by: Marius Vlad marius.vlad@collabora.com