AthenaPoolCnvSvc: Drop hardcoded file size limit of 15 GB
As discussed in ATEAM-776, this MR drops the hardcoded file size limit of 15 GB. The functionality to force a file size limit is still there but AthenaPoolCnvSvc no longer complains if the user asks for a size larger than 15 GB.
With this change we don't need to do ad-hoc things as in PyJobTransforms/DisableFileSizeLimit.py. There are a handful of places in the (old) configuration where the default is overwritten, e.g. merge skeletons in SimuJobTransforms but I haven't touched those in this MR.