Remove xarray_internal concat algorithm in VCF reader

[tomwhite]

As discussed in #665, concat_algorithm="xarray_internal" has scaling issues, and doesn't handle variable-length strings well, so we can drop it to simplify the code.

[hammer]

@tomwhite could you link to the upstream issue for the scaling issues here? 🙏

[tomwhite]

The Dask issue is dask/dask#6745. It's now closed, although the problem hasn't been fixed.

[tomwhite]

I should add that fixing this would avoid another problem we have been having with Xarray: pydata/xarray#5769