glib/gobject.meta: remove slot initargs before call-next-method in make-instance

glib/gobject.meta: remove slot initargs before call-next-method in make-instance

If slot initargs are not removed then shared-initialize will try to set
corresponding slots' values. For constructor-only properties this will throw
warnings, and for other properties this will set them two times (e.g., redundantly).

Initargs are only removed from make-instance's primary method. :before, :after and
:around methods for initialize-instance will receive full arguments list.