No need to use `reference_name` in merge key

[samanvp]

As we shard variants per chromosome before conducting merge stage, we don't need to use reference_name in the merge key. In other words, since we merge variants of each chromosome independently from other chromosomes, there is no need to use reference_name in the key here:

gcp-variant-transforms/gcp_variant_transforms/libs/variant_merge/merge_with_non_variants_strategy.py

Line 84 in 1def29f

yield ':'.join([str(x) for x in [variant.reference_name, i]])

and here:

gcp-variant-transforms/gcp_variant_transforms/libs/variant_merge/move_to_calls_strategy.py

Line 144 in 1def29f

variant.reference_name or '',