build: Drop use of G_DISABLE_DEPRECATED from the build system

It’s no longer used in any of the headers. See preceding commits.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

docs/reference/glib/compiling.xml

@@ -93,11 +93,10 @@ this range will trigger compiler warnings.
 </para>
 
 <para>
-The older deprecation mechanism of hiding deprecated interfaces
+Since GLib 2.62, the older deprecation mechanism of hiding deprecated interfaces
 entirely from the compiler by using the preprocessor symbol
-G_DISABLE_DEPRECATED is still used for deprecated macros,
-enumeration values, etc. To detect uses of these in your code,
-use the commandline option <literal>-DG_DISABLE_DEPRECATED</literal>.
+<literal>G_DISABLE_DEPRECATED</literal> has been removed. All deprecations
+are now handled using the above mechanism.
 </para>
 
 <para>

docs/reference/meson.build

@@ -26,7 +26,6 @@ foreach version : stable_2_series_versions
 endforeach
 
 gtkdoc_common_scan_args = [
-  '--deprecated-guards=G_DISABLE_DEPRECATED',
   '--ignore-decorators=' + '|'.join(ignore_decorators),
 ]