Remove workaround for ROOT/cling problem with Boost small_vector

Cling fails to compile boost::container::small_vector under some circumstances (not reproducible in a standalone test) as seen in lhcb-lcg-dev3/1176 and previously in master.

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In file included from /workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Event/RecVertex_v2.h:18:
In file included from /workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/EventLocalAllocator.h:12:
In file included from /workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/EventContextExt.h:12:
In file included from /workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/EventLocalResource.h:34:
/workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/MonotonicArena.h:69:23: error: too few template arguments for class template 'small_vector'
    boost::container::small_vector<gsl::span<std::byte>, 1, void> m_all_blocks;
                      ^
/cvmfs/sft-nightlies.cern.ch/lcg/nightlies/dev3/Fri/Boost/1.72.0/x86_64-centos7-gcc9-opt/include/boost/container/small_vector.hpp:548:7: note: template is declared here
class small_vector : public small_vector_base<T, Allocator, Options>
      ^
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In file included from /workspace/build/Rec/InstallArea/x86_64-centos7-gcc9-opt/include/LoKi/LoKiTrack_dct.h:20:
In file included from /workspace/build/Rec/InstallArea/x86_64-centos7-gcc9-opt/include/LoKi/LoKiTrack.h:19:
In file included from /workspace/build/Rec/InstallArea/x86_64-centos7-gcc9-opt/include/LoKi/TrSources.h:30:
In file included from /workspace/build/Rec/InstallArea/x86_64-centos7-gcc9-opt/include/LoKi/TrackTypes.h:24:
In file included from /workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Event/RecVertex_v2.h:18:
/workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/EventLocalAllocator.h:69:52: error: expected ')'
          return reinterpret_cast<T*>( m_resource->template allocate<alignof( T )>( n * sizeof( T ) ) );
                                                   ^
/workspace/build/LHCb/InstallArea/x86_64-centos7-gcc9-opt/include/Kernel/EventLocalAllocator.h:69:38: note: to match this '('
          return reinterpret_cast<T*>( m_resource->template allocate<alignof( T )>( n * sizeof( T ) ) );

The workaround is to explicitly set the template arguments with defaults. At some point we should remove the workaround when ROOT is fixed