go.mod,go.sum: ran go mod tidy

Change-Id: I7cb2cbfa31016a400970eb1fdba0503d66b1b847
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285836
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>