Remove BLAS from linfa-clustering

[YuhanLiin]

Note that the only part of linfa-clustering that uses BLAS routines is Gaussian Mixture, which didn't actually show a difference in the benchmarks. For other modules, BLAS only changes the matrix multiplication algorithm. BLAS only speeds up the appx_dbscan benchmark, but actually slows down several other benchmarks. As such, BLAS doesn't actually benefit linfa-clustering, so it should be removed.

The linfa-clustering benchmarks are linked in this comment.

[oojo12]

I think we can close this now, per the above merge. My bad for not properly linking that PR to this issue.