runtime: remove tracebackinit and unused skipPC
CL [152537](https://go-review.googlesource.com/c/go/+/152537/) changed the way inlined frames are represented in tracebacks to no longer use skipPC
Change-Id: I42386fdcc5cf72f3c122e789b6af9cbd0c6bed4b
GitHub-Last-Rev:
79c26dcd532907eda4ffc30951845c1c01243501
GitHub-Pull-Request: golang/go#39829
Reviewed-on: https://go-review.googlesource.com/c/go/+/239701
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>