mountoperation: Free user_widgets list upon dialog destruction

There's a list user_widgets that contains all of the entries and
selections during authentication.  This is only freed  upon
finalizing the GtkMountOperation.  It's possible (and true for the
GVFS SMB implementation) that a MountOperation can have the
gtk_mount_operation_ask_password_do_gtk () function called multiple
times (i.e. bad password).  The user_widgets list grows with now
invalid pointers to old widgets (causing unexpected behavior and
seg faults).

Free the user_widgets list upon dialog destruction, we don't need it
anymore.

Fixes: GNOME/gtk#5059
Fixes: GNOME/gtk#5058