How do I delete a saved wifi network, when unable to use the Network panel/GUI in System pref?
You can check the list of preferred networks with the command: 

sudo networksetup -listpreferredwirelessnetworks en1

You can then copy the name of the wifi you want to remove from the list and use the command (replacing wifi_to_be_removed with the name of the wifi):

sudo networksetup -removepreferredwirelessnetwork en1 wifi_to_be_removed

Note: en1 is usually the device name of the single wifi card in a mac that also has a ethernet port. To be sure of the device name use the command: sudo networksetup -listallhardwareports
to list the various network devices and their denomination.