Trigger: Remove unused libraries and public/private header cleanup

Merged Frank Winklmeier requested to merge fwinkl/athena:trigalg_lib_cleanup into master

Library and public/private cleanup of trigger packages in TrigAlgorithms and TrigHypothesis:

  • Remove unused library (atlas_add_library) targets
  • Move private header files to src directory
  • Delete some obsolete files

NB: Even though there are many commits (one per package), please do not squash.

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