Revert "Guess vcf.gz index if ommitted -- special case for VCF files, as nearly all are indexed."
Revert "Guess vcf.gz index if ommitted -- special case for VCF files, as nearly all are indexed."

This reverts commit 1225a84470dc148a1685c26e63d24e0b58b29e1e.

# Conflicts:
#	js/feature/featureFileReader.js

igvteam/igv.js,igvteam/igv.js