Remove event mixing test, which goes nowhere

Author: saganatt

Framework/Core/CMakeLists.txt

@@ -220,7 +220,6 @@ add_executable(o2-test-framework-core
               test/test_DeviceStateHelpers.cxx
               test/test_Expressions.cxx
               test/test_ExternalFairMQDeviceProxy.cxx
-              test/test_EventMixing.cxx
               test/test_FairMQOptionsRetriever.cxx
               test/test_FairMQResizableBuffer.cxx
               test/test_FairMQ.cxx