xerrors: Revert "xerrors: redirect to go1.13 primitives for 1.13"

This reverts CL 167577

Reason for revert:
errors.Frame and errors.Formatter were removed in CL 176997

Fixes golang/go#32246

Change-Id: I521cc1176311721a18aaf76d4966e945c8453e9d
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/177379
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Joiner <anacrolix@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
6 files changed