network: fix NMConnectionSection destruction

The patch fixes the following warning, and along with it, the proper
destruction of the NMConnectionSection is performed so that items get
correctly removed from the menu.

(gnome-shell:24528): Gjs-WARNING **: JS ERROR: TypeError:
this.statusItem is undefined
NMConnectionSection<.destroy@/home/aleksander/gnome/install/share/gnome-shell/js/ui/status/network.js:173
wrapper@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:213
_parent@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:175
NMConnectionDevice<.destroy@/home/aleksander/gnome/install/share/gnome-shell/js/ui/status/network.js:292
wrapper@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:213
_parent@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:175
NMDeviceModem<.destroy@/home/aleksander/gnome/install/share/gnome-shell/js/ui/status/network.js:448
wrapper@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:213
NMApplet<._removeDeviceWrapper@/home/aleksander/gnome/install/share/gnome-shell/js/ui/status/network.js:1421
wrapper@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:213
NMApplet<._deviceRemoved@/home/aleksander/gnome/install/share/gnome-shell/js/ui/status/network.js:1416
wrapper@/home/aleksander/gnome/install/share/gjs-1.0/lang.js:213

https://bugzilla.gnome.org/show_bug.cgi?id=709248