build: Remove all remaining gtk-doc build infrastructure

It’s no longer needed: the documentation is built by gi-docgen now.

Fixes: #3037

docs/reference/gobject/meson.build

@@ -1,57 +1,3 @@
-if get_option('gtk_doc')
-  subdir('xml')
-
-  ignore_headers = [
-    'tests',
-    'gatomicarray.h',
-    'gobject_probes.h',
-    'gobject_trace.h',
-    'gtype-private.h',
-    'glib-enumtypes.h',
-    'gobject-visibility.h',
-  ]
-
-  docpath = join_paths(glib_datadir, 'gtk-doc', 'html')
-  version_conf = configuration_data()
-  version_conf.set('GLIB_VERSION', meson.project_version())
-  configure_file(
-    input: 'version.xml.in',
-    output: 'version.xml',
-    configuration: version_conf
-  )
-
-  gtkdocincl = include_directories('.')
-
-  gnome.gtkdoc('gobject',
-    main_xml : 'gobject-docs.xml',
-    namespace : 'g',
-    mode : 'none',
-    dependencies : [libgobject_dep, libglib_dep],
-    include_directories : [gtkdocincl],
-    src_dir : 'gobject',
-    scan_args : [
-      '--ignore-decorators=' + '|'.join(ignore_decorators.replace('GLIB', 'GOBJECT')),
-      '--rebuild-types',
-      '--ignore-headers=' + ' '.join(ignore_headers),
-    ],
-    content_files : [
-      'glib-mkenums.xml',
-      'glib-genmarshal.xml',
-      'gobject-query.xml',
-      'tut_tools.xml'
-    ],
-    html_assets : [
-      'images/glue.png'
-    ],
-    fixxref_args: [
-      '--html-dir=' + docpath,
-      '--extra-dir=' + join_paths('gobject', '..', 'glib', 'html'),
-    ],
-    install: true,
-    check: false,
-  )
-endif
-
 if get_option('man')
   manpages = ['glib-mkenums', 'glib-genmarshal', 'gobject-query']
   foreach page : manpages
@@ -64,7 +10,6 @@ if get_option('man')
   endforeach
 endif
 
-# gi-docgen version
 expand_content_files = [
   'boxed.md',
   'concepts.md',