AnalysisBase: Add dummy flake8 configuration

flake8 is not available in AnalysisBase. While our cmake configuration does handle this correctly, it fails once developers start using special flake8 options, e.g.:

... POST_BUILD_CMD ${ATLAS_FLAKE8} --enable-extensions ATL902

Even if ATLAS_FLAKE8 is not defined it will now try to run a command of name --enable-extensions, which of course fails. An easy workaround is to set( ATLAS_FLAKE8 true ) where true here is the shell builtin, which returns success and swallows any command line args given to it.

Also in this MR: Remove a useless flake8 test in TrigBunchCrossingTool.

cc @akraszna