VxVertex clean up CMakeList and move large methods to .cxx

Christos Anastopoulosrequested to merge
ATLAS-EGamma/athena:vxVertex_move_large_more_than_few_lines_to_cxx into master
  • Remove un-needed atlas_depends_on_subdirs
  • Keep only the "trivial" setter/getters as inline in the .icc

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