Hardcoded flag for compute capability 2.0

[sbastrakov]

This line has a hardcoded flag for compute capability 2.0, which is not supported since CUDA 9. So examples do not compile with the compiler stack used by Alpaka and PIConGPU.
The cmake file generally has room for improvement, but this is the most immediate issue.

[ax3l]

Little regression noted in #164 (comment)

Re-opening to keep track of it, so it does not get lost.

[psychocoderHPC]

Thanks for re-open it. The dev branch is the default branch, therefore, this issue was auto closed.

[sbastrakov]

@ax3l thanks, will do. However, not sure how that was a regression: a hardcoded value replaced with a slightly modernized hardcoded value.

[ax3l]

Thanks for fixing!