Update BCFtools to use stable sort

[ASLeonard]

Was looking around at some bcftools sort code because of the CI fail in #1809, and noticed that in bcftools v1.21 the sort function was made stable (although it doesn't appear in the release notes, see samtools/bcftools#2252). I tested this builds in docker and am currently running it on a previous graph to see if it works, as making sure the cactus_call is right is a bit finickity.

As a separate note, the samtools/htslib in downloadMafTools didn't appear to do anything (although it exports HTSLIB variable), so I removed that and nothing broke (hopefully docker didn't cache anything that would stop this breaking). This seems to save a reasonable amount of build time as htslib is big.

[glennhickey]

Thanks for this. Very interesting about bcftools sort being stable now. I've triggered the CI tests.

The htslib compilation in downloadMafTools is indeed not necessary for the docker build. Or, I guess, on a system with htslib installed. But it is required for the static binary build (try it yourself ./downloadMafTools 1), or on a system without htslib.