remove dead code (detect_libstdcxx_version and check_libstdcxx_version) ?

[t-bltg]

It looks like BinaryBuilder.Auditor.detect_libstdcxx_version and BinaryBuilder.Auditor.check_libstdcxx_version is dead code and not used anywhere in BinaryBuilder, also without any traces of usage anywhere else in the julia ecosystem:
https://github.com/JuliaPackaging/BinaryBuilder.jl/blob/master/src/auditor/compiler_abi.jl#L107-L165

So should they be removed ?

Hoisted out of #1392.

[giordano]

I never touched this code because I always assumed this was meant to be used at some point, since Julia itself also has mechanisms to detect the libstdc++ version (on Linux do Base.BinaryPlatform.HostPlatform()).

[t-bltg]

Ok, I wanted to add tests covering these lines, but they were failing, so I'm not sure what to do here.