strings: move TrimPrefix and TrimSuffix to stringslite
To help packages use these functions like "os" which using
the copied function "stringsTrimSuffix".
Change-Id: I223028ed264c7b7e95534b4883223af0988cda68
GitHub-Last-Rev:
2fd8fbf5286e5a4abdb03704d69f02e32d3f1a6b
GitHub-Pull-Request: golang/go#67151
Reviewed-on: https://go-review.googlesource.com/c/go/+/583075
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>