TriggerJobOpts: disable HepMcParticleLink warnings in runHLT_standalone

Fixes ATR-21838, i.e. suppresses thousands of warnings like this:

HepMcParticleLink                          0   0 WARNING cptr: Mc Truth not stored for event with event number 470

in Trigger jobs running on MC.

The effect on a 20-event simple trigger test (checked on test_trig_mc_v1Phys_build.py) is removing around 80000 of these warnings and reducing the log size from 15 MB to 6 MB.