switch to using VEP cache from AoD instead of maintaining our own mirror

[allaway]

I discovered today that the past several VEP cache tarballs are available from amazon open data, e.g. s3://hail-vep-pipeline/vep/cache/homo_sapiens_vep_112_GRCh38.tar.gz

I did confirm that this mirror matches the checksum (using sum) of the official vep repository, at least for version 112 of the vep cache: https://ftp.ensembl.org/pub/release-112/variation/indexed_vep_cache/CHECKSUMS

This bucket is also in us-east-1 and is part of the amazon open data program, so it doesn't make sense for us to maintain a copy. This also makes it much easier for us to update to different VEP versions in the future: https://registry.opendata.aws/hail-vep-pipeline/