encoding/json/jsontext: remove ineffectual subtraction

The i-0 might make sense if there was symmetry elsewhere
in the same function, but it looks odd on its own.

Change-Id: Ie7b7c94968ca19fed842212387659f0338caae3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/775301
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>