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: 2fd8fbf528
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>

src/internal/stringslite/strings.go

@@ -122,3 +122,17 @@ func CutSuffix(s, suffix string) (before string, found bool) {
 	}
 	return s[:len(s)-len(suffix)], true
 }
+
+func TrimPrefix(s, prefix string) string {
+	if HasPrefix(s, prefix) {
+		return s[len(prefix):]
+	}
+	return s
+}
+
+func TrimSuffix(s, suffix string) string {
+	if HasSuffix(s, suffix) {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}

src/strings/strings.go

@@ -1075,19 +1075,13 @@ func TrimSpace(s string) string {
 // TrimPrefix returns s without the provided leading prefix string.
 // If s doesn't start with prefix, s is returned unchanged.
 func TrimPrefix(s, prefix string) string {
-	if HasPrefix(s, prefix) {
+	return stringslite.TrimPrefix(s, prefix)
-		return s[len(prefix):]
-	}
-	return s
 }
 
 // TrimSuffix returns s without the provided trailing suffix string.
 // If s doesn't end with suffix, s is returned unchanged.
 func TrimSuffix(s, suffix string) string {
-	if HasSuffix(s, suffix) {
+	return stringslite.TrimSuffix(s, suffix)
-		return s[:len(s)-len(suffix)]
-	}
-	return s
 }
 
 // Replace returns a copy of the string s with the first n