nlohmann json: Remove version 2.1.1
nlohmann json: Remove version 2.1.1

This is an old version which had a different directory structure
and as a result different include paths. Since this version is
quite old, I don't think it adds much value.
Removing it to avoid confusion among users.

mattgodbolt/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer