Assimp support broken with assimp 5.0

[trissim]

On arch linux, compiling with -DBRAYNS_ASSIMP_ENABLED=ON using assimp 5.0 fails.
Replacing assimp 5.0 with a manual installation of assimp 4.1 fixes the issue.

Compilation with assimp 4.1 produces the following error:

/home/ts/programs/Braynssrc/Brayns/brayns/io/MeshLoader.cpp: In function ‘Assimp::Importer brayns::{anonymous}::createImporter(const brayns::LoaderProgress&, const string&)’:
/home/ts/programs/Braynssrc/Brayns/brayns/io/MeshLoader.cpp:119:12: error: use of deleted function ‘Assimp::Importer::Importer(const Assimp::Importer&)’
  119 |     return importer;
      |            ^~~~~~~~
In file included from /home/ts/programs/Braynssrc/Brayns/brayns/io/MeshLoader.cpp:23:
/usr/include/assimp/Importer.hpp:140:5: note: declared here
  140 |     Importer(const Importer& other)=delete;

[Nadir Román Guerrero]

This issue is now fixed in latest realease (which supports up to Assimp 5.0.1). Moreover, assimp has been removed from brayns for future releases. Closing.