display: Stop night light dialog being destroyed twice

The night light dialog is both marked as "destroy_with_parent" and explicitly
destroyed in the panel. Drop one of these.

Causes the warning after opening the dialog then closing the app:
(gnome-control-center:19887): Gtk-CRITICAL **: 11:00:01.370: gtk_widget_destroy: assertion 'GTK_IS_WIDGET (widget)' failed

This bug was introduced in ed36688c
11 jobs for display-nightlight-dialog-dispose in 6 minutes and 32 seconds (queued for 5 minutes and 42 seconds)
Status Job ID Name Coverage
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passed #126019
build

00:02:19

 
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passed #126021
asan

00:02:17

passed #126020
test

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manual #126026
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flatpak
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ppc64le
manual #126023
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