remove CMake `alpaka_fetchcontent_finalize`

[SimeonEhrig]

During PR #377 alpaka_fetchcontent_finalize was deprecated. Should be removed before release.

[psychocoderHPC]

I reopened this issue because the method should be removed from CMakeLists.txt but before we should update https://enccs.github.io/gpu-programming/8-portable-kernel-models/#id3