NMConnectionSection: don't remove connections that were never added

NMApplet will call removeConnection() unconditionally on all sections,
including those that had nothing to do with the connection in the first
place.

Fixes:

Gjs-WARNING **: JS ERROR: TypeError: this._connectionItems.get(...) is undefined
NMConnectionSection<.removeConnection@resource:///org/gnome/shell/ui/status/network.js:323
wrapper@resource:///org/gnome/gjs/modules/lang.js:169
NMApplet<._connectionRemoved@resource:///org/gnome/shell/ui/status/network.js:1885
wrapper@resource:///org/gnome/gjs/modules/lang.js:169

https://bugzilla.gnome.org/show_bug.cgi?id=725958