Reference data includes bwa indices under incorrect name

[unode]

Since ngless 0.10.0 index files created by bwa are versioned reference.fa-bwa-0.7.17.gz.bwt instead of reference.fa.gz.bwt.

The bundled reference genomes (hg19, mm10.p5, ...) include bwa indices but only in the non-versioned format.
As such, we should either create versioned indices (or symlinks to the appropriate versions) or delete the indices and let them be created by ngless.

In the long run we may want to delete indices entirely as it simplifies packaging and distribution of reference data for every new ngless version. As-is, leaving the indices in the reference package only wastes space on the users' end.

[luispedro]

I'm not sure whether the format of the indices ever changed in bwa, but I can see how maybe we should try to stay on the safe side.

[unode]

bwa hasn't, minimap2 did at some point, but I agree that the change in 0.10.0 was mostly out of caution.