Altium - broken unconnected nets
As these traces are not assigned to any net, they are not deleted by "Design -> Netlist -> Clear all nets".
There are two possibilities to do so:


Use "Route » Un-Route » All", which clears all physical connections on your PCB (https://www.altium.com/documentation/17.0/display/ADES/PCB_Cmd-UnRoute((UnRoute))_AD)
Right click a trace -> find similar objects and select the layer and switch to "Same". This selects all tracks on the specified layer and you can easily delete them