The filetype of a routing ASM is the same as for a 'normal' ASM, namely .SLDASM.
There for changing file types is not the answer.
When you want to make use of routing the regular course of action is to start an ASM and then use the methods at hand to start the actual routing, be it starting from a point or with a routing part. Either way means however that there will be a sub-ASM created in the main ASM This sub-ASM holds the actual routing.
When you say that you deleted all the cables, do you mean that you deleted all the cable parts from the folder Route parts?
If so, then please realize that it only means that next time that you try to do an edit route it will simply rebuild those cable parts.
If your goal is to have the route parts (the elbows and such) then your best bet is to change all parts of the 3dsketch, which aren't elbow ,to construction line
This should leave you will all the parts, and without any of the cables.
It becomes a defacto non-routing ASM.
Hope this helps