HLTMPPU: Clean up logging messages to stdout/stderr
Some cleanups for the messages that the HLTMPPU prints to make the output for regular users a bit less overwhelming:
- Use
ERS_LOG
if possible - Remove duplicate messages
- Use stderr only for warnings/errors but not for regular messages
There is probably more cleanup that could be done but for the purpose of what's used/seen in athenaHLT.py
this should be already much better than before.