Remove cblas/atlas implementations
This was a feature in v0.0.2 but with MKL dropped as a feature, building atlas an unfeasible task due to its single threaded nature and cblas always being inferior, openblas is the only viable alternative right now.
In addition, cblas takes away precious CI resources. Therefore, drop cblas/atlas completely. We still retain the libreml-blas stack, in case in the future there is an additional open source BLAS implementation.