DRAWCommonByteStream.py: Delete (effectivly overwrite) per-existing DRAW files

POOL files created by an athena jobs are siliently overwritten if the same job is run again while pre-existing DRAW files create a job failure at a late stage in the job. This MR synchronises the behavoir of DRAW output with POOL output by deleting any pre-existing DRAW files.

Re-running the same job in the same directory is common during debugging sessions, and having the job fail at a late stage because of a lingering DRAW file is very annoying.