Removing / with tangent denominator

along the same lines as #643
We should not have e.g.
```julia
Base.:(/)(a, b::AbstractThunk) = a / unthunk(b) 
Base.:(/)(a::AbstractThunk, b::AbstractThunk) = unthunk(a) / unthunk(b)
```
We have a few of these 

Since while it makes sense to divide a tangent by a scalar (its just scaling down) it does done make sense to divide a scalar by a tangent. 