As of golang/go#50827, gopls no longer needs to build at Go 1.12. This
was the only reason to continue using xerrors in the jsonrpc2 libraries.
Remove this usage as a step toward eliminating the xerrors dependency
from x/tools.
For golang/go#52442
Change-Id: I1a0a1bbf57e6606c66b99b27439adf6f65c26a60
Reviewed-on: https://go-review.googlesource.com/c/tools/+/401155
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
