http://golang.org/cl/7623 refactored how line history works and
introduced a new TrimPathPrefix field to replace the existing Trimpath
field, but never removed the latter or updated its users.

Fixes #10503.

Change-Id: Ief90a55b6cef2e8062b59856a4c7dcc0df01d3f2
Reviewed-on: https://go-review.googlesource.com/9113Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>