Remove panic wrapper in tracing callback

[vcfxb]

This was there when I last modified the tracing code and I left it in without checking on it but looking again at it now, it seems like it should be removed in favor of rust's native extern "C" panic handling.

This is arguably a bugfix, so it does not require cutting a new release in my opinion, although you may want to anyway since there's a potential for user's code to start panicking/aborting at runtime where it wasn't before.