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>

doc/sphinx/meson.build

@@ -56,13 +56,6 @@ script_doxy_sphinx = configure_file(
 	configuration: script_data
 )
 
-doxygen_target = custom_target(
-			'weston-doc-doxygen',
-			command: [ doxygen, doxygen_conf_weston ],
-			output: 'doxygen_doc',
-			build_by_default: false
-)
-
 # copy everything to build_dir, if you plan on adding other files in the top
 # rootdir of sourcedir, please add them here as well, otherwise use 'toc/'s
 # meson.build file
@@ -79,7 +72,6 @@ sphinx_doc = custom_target(
 		command: script_doxy_sphinx,
 		output: 'doc',
 		build_by_default: true,
-		depends: doxygen_target
 )
 
 # we need this because we will have a stale 'doc' directory