SimKernelMT: avoid separate ToolHandle for ParticleKiller (ATLASSIM-6535)

John Derek Chapmanrequested to merge
jchapman/athena:ATLASSIM-6535_23.0 into 23.0

This refactoring avoids spurious warnings about there being two instance of a particle killer simulator tool in the job. Should have no affect on the simulation output. Closes ATLASSIM-6535

Tagging @cyoung

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