Remove workaround for gzip/BGZF compressed VCF headers

[heuermh]

Remove the workaround here

https://github.com/bigdatagenomics/adam/blob/master/adam-core/src/main/scala/org/bdgenomics/adam/rdd/ADAMContext.scala#L159

for gzip/BGZF compressed VCF headers. The issue has been fixed upstream

Fix VCFHeaderReader for gzipped and BGZF VCF formats
HadoopGenomics/Hadoop-BAM#97

Note this will require bumping the dependency version of Hadoop-BAM to 7.5.1 (or 7.6.0) which may bring along a transitive dependency version bump of htsjdk to 2.5.0, see HadoopGenomics/Hadoop-BAM#107.

[heuermh]

Fixed by #1060