Remove dependency on internal htslib file

Reapply 9f0acb8bffe4a872f0d76768df284cd3b3bba5ad's removal of unneeded
$(HTSDIR)/version.h dependency, which was mistakenly omitted by
3368b1f960c136fca4d456342bb615af18c194ea.  Depending on this file is
unnecessary and actively wrong because it won't exist when building
against an installed htslib; cf. Homebrew/homebrew-science#1015.