Fix Collada loading on Windows

Resolves #8017 (closed) by reverting part of !2972 (merged). boost::filesystem is a dependency of Collada DOM, so we can't load Collada meshes if it's not available at runtime. We can't do anything about this except provide the DLL like we used to. The problem shouldn't affect other platforms as installing Collada DOM will implicitly install boost::filesystem if you're using a package manager.

This needs to be merged before 0.49 is branched or we'll have yet another release where Collada meshes don't work.