st: Remove invalid introspection annotation

(optional) is only valid for (out) or (inout) parameters (that are
marked as such).

However GError** arguments appear as throws="1" in the GIR anyway
instead of an explicit parameter, so we don't need any annotation
at all here.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1408

src/st/st-theme.c

@@ -254,7 +254,7 @@ insert_stylesheet (StTheme      *theme,
  * st_theme_load_stylesheet:
  * @theme: a #StTheme
  * @file: a #GFile
- * @error: (optional): a #GError
+ * @error: a #GError
  *
  * Load the stylesheet associated with @file.
  *