Do not arbitrarily release Python GIL in HLTControlFlowMgr

!3340 (merged) introduced an unconditional release of Python GIL while the problem should have been handled in Gaudi as done in gaudi/Gaudi!1369 (merged) and gaudi/Gaudi!1387 (merged). Moreover the unconditional release of the GIL prevent running the HLTControlFlowMgr from Gaudi.exe (for example in test workflows like gaudirun.py -n -o myopts.opts some_complicated_options.py && Gaudi.exe myopts.opts).