TrigConfiguration: CMake cleanup, flake8 checking

Frank Winklmeierrequested to merge
fwinkl/athena:trigconf_cmake into master

Cleanup CMake configuration of all packages under Trigger/TrigConfiguration:

  • remove atlas_depends_on_subdirs
  • enable flake8 checking where applicable and make code compliant

Also fix Py3 issues and delete some obsolete files.

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