Remove duplicate friend declarations

Fix after rebase

Author: franzpoeschel

include/openPMD/backend/Attributable.hpp

@@ -131,7 +131,6 @@ class Attributable
     friend struct traits::GenerationPolicy;
     friend class Iteration;
     friend class Series;
-    friend class Series;
     friend class Writable;
     friend class WriteIterations;
 

include/openPMD/backend/BaseRecord.hpp

@@ -56,9 +56,7 @@ class BaseRecord : public Container< T_elem >
     friend class ParticleSpecies;
     friend class PatchRecord;
     friend class Record;
-
     friend class Mesh;
-    friend class ParticleSpecies;
 
     std::shared_ptr< internal::BaseRecordData< T_elem > > m_baseRecordData{
         new internal::BaseRecordData< T_elem >() };

include/openPMD/backend/Container.hpp

@@ -135,7 +135,6 @@ class Container : public Attributable
     friend class ParticlePatches;
     friend class internal::SeriesData;
     friend class Series;
-    friend class Series;
     template< typename > friend class internal::EraseStaleEntries;
 
 protected: