Remove pointer to previous Python frame on Traceback frame.

[copybara-service[bot]]

Remove pointer to previous Python frame on Traceback frame.

This removes Traceback->Frame->f_back.

The context for this change: I want to attach Tracebacks to JAX run-time errors (checkify) that point to the source location where the error happened. Python tracebacks have frames which keep a reference to their locals. These locals can be tracers, which result in tracer leaks. xla Traceback creates its own frame without locals, but still keeps a reference to the previous frame which is a regular Python frame (with a reference to its locals).