SLATE package: fix variant/header errors (#11353)
SLATE package: fix variant/header errors (#11353)

* The 'mkl' variant implementation was not complete and was
  preventing the package from building, so remove it.
* The slate.hh file has moved in the 'develop' version (which is
  currently the only version of SLATE listed)
LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack