Variant: Don't refsink variants created using the custom cast ctor.

	* glib/src/variant.ccg:
	* glib/src/variant.hg:
	* glib/src/variant_basictypes.cc.m4: Remove the call to
	g_variant_ref_sink() in the create() methods of the variant types that
	use the custom cast constructor to wrap the newly created C object
	because the custom cast constructor already refsinks the object if
	necessary.