21.2 MuonSelectionTool small cleanup

Magnar Kopangen Buggerequested to merge
mkbugge/athena:21.2-MSTsmallCleanup into 21.2

Small cleanup/maintenance of the MuonSelectionTool. In particular:

  • Added missing initializers in copy constructor
  • Moved to consistent use of unique_pointer
  • Removed unnecessary "using namespace xAOD" and unused variables

Tagging @szambito and @nkoehler

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