Remove setting of CoreDumpSvc.FatalHandler

With athenaMT it is best to rely on the default (ROOT) signal handler in CoreDumpSvc, which uses gdb to produce the stacktrace for all threads. Setting FatalHandler = 438 makes use of the SealSignal handler, which only produces one trace. For production environments where the gdb traces are not needed, one should override the setting in the transform or AMI tag as is already done today.

Relates to ATEAM-629.