Suppress clang warnings

Some trivial changes to suppress some clang warnings recently introduced in master

../Event/RecEvent/include/Event/PrimaryVertices.h:193:29: warning: explicitly defaulted copy assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
../Event/TrackEvent/include/Event/PrVeloTracks.h:55:3: warning: 'Tracks' defined as a struct here but previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]

Event/RecEvent/include/Event/PrimaryVertices.h

@@ -18,7 +18,7 @@
 
 // Forward declaration
 namespace LHCb::Pr::Velo {
-  class Tracks;
+  struct Tracks;
 }
 
 namespace LHCb::Event::PV {
@@ -189,8 +189,6 @@ namespace LHCb::Event::PV {
         , tracks{Zipping::generateZipIdentifier(), rhs.tracks}
         , velotrackmap{rhs.velotrackmap} {}
     PrimaryVertexContainer( PrimaryVertexContainer&& ) = default;
-    PrimaryVertexContainer& operator=( PrimaryVertexContainer&& rhs ) = default;
-    PrimaryVertexContainer& operator=( const PrimaryVertexContainer& rhs ) = default;
     /// Forwarding functions to make this look like a container of vertices.
     auto        begin() const { return vertices.begin(); }
     auto        end() const { return vertices.end(); }