gio: Remove unnecessary appinfo_sources variable

There was no obvious logical need to list the `GAppInfo` subclass
sources separately in the build. It makes more sense to add them to the
platform-specific source lists, since they are platform specific.

This will be used in an upcoming commit which generates
platform-specific GIR files, so needs the full platform-specific lists
of sources.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

girepository/introspection/meson.build

@@ -111,7 +111,6 @@ gio_gir_sources = [
   gio_base_sources,
   application_sources,
   gdbus_sources,
-  appinfo_sources,
   contenttype_sources,
   unix_sources,
   win32_sources,