libgimpwidgets: do not free the GtkAdjustment when finalizing...
... GimpMemsizeEntry.
A GtkAdjustment is a GInitiallyUnowned, which means it is created as a
floating reference. GtkSpinButton assumes its ownership by calling
g_object_ref_sink() in gtk_spin_button_new() implementation. Thus it
will take care of freeing it and when we try to unref it in finalize(),
the object already doesn't exist.
Alternatively we could keep another ref (by calling g_object_ref_sink()
ourselves) but since the spin button is a child of the entry, it will
live all the way until the entry is freed. There is no need to increment
the references. So instead, simplify the code, and don't try to free an
object we don't own anymore.
This fixes CRITICAL assertions:
> g_object_unref: assertion 'G_IS_OBJECT (object)' failed