argFile::_mergeArgs(). Make sure --threads is not passed to merger in hybrid MP+MT jobs

Vakhtang Tsulaiarequested to merge
tsulaia/athena:23.0-hybrid-merge into 23.0

Fixes the crash in the merge step of hybrid MP+MT caused by the fact that the merger is configured to run athenaMT, while the number of concurrent events in runArgs is set to 0

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