Remove "heterogeneous" where it doesn't make sense

[TApplencourt]

We agreed that the restriction of "heterogeneous" devices makes little sense. For example, this means that pure-OpenMP implementations are not "backend"

I guess we should clarify that cycle provides a portable abstracted discrete device concept that can be used regardless of the physical hardware (discrete, integrated, same ISA, or heterogenous...). But I was not smart enough to put it in the spec, so this pr just removes heterogeneous from a few places.

[TApplencourt]

Put back the can as it opens a new can of worms. We will deal with it in another PR.

[tomdeakin]

WG approved to merge.

[TApplencourt]

Thanks guys!