net: use bytealg.LastIndexByteString
There is no need to handwrite the "last" function, the bytealg package already provides "LastIndexByteString".
Change-Id: I6000705bffe8450a10cf8f3fa716a8d4605ada1f
GitHub-Last-Rev:
6627c65fb40fad96239edd28bde27a30f9f8f544
GitHub-Pull-Request: golang/go#62647
Reviewed-on: https://go-review.googlesource.com/c/go/+/527976
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>