Remove kron functionality

[maleadt]

... once we depend on the next version of GPUArrays, where @albertomercurio has implemented a generic version: JuliaGPU/GPUArrays.jl#575

[kshyatt]

Have a PR ready to go for this, just waiting on a new GPUArrays tag.

[maleadt]

Let's maybe wait until JuliaGPU/GPUArrays.jl#576 lands before doing so.