Remove arrow file generation when vaex 4.0.0 is available

[ajstewart]

vaex 4.0.0 includes the ability to open parquet files in an out of core context. This means that the arrow file will no longer be required when this is released.

However from testing it is still beneficial to create a measurements.arrow file that contains all the association information already merged - as this can still take a few minutes to generate with vaex, but is negligible in the context of a pipeline run.

vaex 4.0.0 is currently at the alpha stage, so will wait for a more stable version.

[ajstewart]

This is closely related to issue askap-vast/vast-tools#225 where I have been testing the performance of vaex 4.0.0 on using the parquets.

From recent testing, the performance of the arrow file is still much better compared to the parquets and hence the arrow file is still beneficial.

So for now this remains on hold.