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>
2 jobs for fix-wrong-mtime in 1 minute and 25 seconds (queued for 4 seconds)
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  Container Prep
passed #391579
container_prep

00:00:26

 
  Build
passed #391580
build-native-meson

00:00:58