HLTScheduler, MDF - Fix Context memory leak

Fixes sanitizer memory leak

Direct leak of 336 byte(s) in 1 object(s) allocated from:
    #0 0x7f583bcb0da8 in operator new(unsigned long) /afs/cern.ch/cms/CAF/CMSCOMM/COMM_ECAL/dkonst/GCC/build/contrib/gcc-8.2.0/src/gcc/8.2.0/libsanitizer/lsan/lsan_interceptors.cc:229
    #1 0x7f580e49a1c9 in virtual thunk to LHCb::MDFSelector::createContext(IEvtSelector::Context*&) const (/workspace/build/LHCb_HEAD/InstallArea/x86_64-centos7-gcc8-dbg+lsan/lib/libMDF.so+0xc51c9)
    #2 0x7f5823819518 in EventSelector::firstOfNextStream(bool, EvtSelectorContext&) const /workspace/build/Gaudi/GaudiCoreSvc/src/EventSelector/EventSelector.cpp:78
    #3 0x7f5823819fb0 in EventSelector::createContext(IEvtSelector::Context*&) const /workspace/build/Gaudi/GaudiCoreSvc/src/EventSelector/EventSelector.cpp:161
    #4 0x7f581b8425ea in HLTControlFlowMgr::nextEvent(int) /workspace/build/LHCb/Hlt/HLTScheduler/src/HLTControlFlowMgr.cpp:398
    #5 0x7f581b85bf1e in virtual thunk to HLTControlFlowMgr::executeRun(int) (/workspace/build/LHCb_HEAD/InstallArea/x86_64-centos7-gcc8-dbg+lsan/lib/libHLTScheduler.so+0xe4f1e)
    #6 0x7f58237cd97c in ApplicationMgr::executeRun(int) /workspace/build/Gaudi/GaudiCoreSvc/src/ApplicationMgr/ApplicationMgr.cpp:824
    #7 0x7f58245771dc in Gaudi::Application::run() /workspace/build/Gaudi/GaudiKernel/src/Lib/Application.cpp:81
    #8 0x7f5824576722 in _py_Gaudi__Application__run /workspace/build/Gaudi/GaudiKernel/src/Lib/Application.cpp:111
    #9 0x7f5835049435 in ffi_call_unix64 (/cvmfs/lhcb.cern.ch/lib/lcg/releases/LCG_96/Python/2.7.16/x86_64-centos7-gcc8-dbg/lib/python2.7/lib-dynload/_ctypes.so+0x1a435)
    #10 0x7ffc94285f1f  ([stack]+0x1df1f)