ATR-24784: fixing a bug introduced previously by deleting the line to fill a variable

I've accidentally introduced a bug by deleting the line in GenerateMenuMT in which self.signaturesToGenerate was filled which caused the test test_trigID_fslrt_rhadron.py to fail which was reported in ATR-24784. This was actually identified by @bkerridg (thank you, really well done and spotted!). I don't think this variable is actually needed anymore, it's being replaced by self.availableSignatures which stores the signatures that need to be imported, directly obtained from the dictionary.