Remove non-existent thresholds & L1 items from emu test in DecisionHandling

In !43855 (merged), a check on the existence of L1 thresholds was implemented. Consequently, the Emu menu/step tests in DecisionHandling broke, but apparently were not active in the corresponding MR's CI. This then caused failures when full unit tests were activated in !43911 (merged).

The missing threshold was EM5, which I have now erased and replaced with EM3 everywhere in the Emu test NewJO config and LS2_emu_v1 menu. There was also no L1 item "MU6_EM3", in fact the only muon+electron L1 is "EM7_MU10". For the emu_step_menu_processing test, this shows some minor changes after I changed the L1 thresholds to match MU6_EM3. I found it odd that the emu_step_processing test did not specify L1 thresholds, but still behaves well -- I'm not sure whether any EM->muv1 MU->ev2 cross-wiring happens, or how to fix it if it does.

Test references had to be updated.

Tagging @fpastore, @tbold in case it is thought that this is not a reasonable solution.