app/appengine: delete unused tail func

It's the only func not used by the template. Looking at the git
history, I don't see it ever being used.

Change-Id: Id074712996589fc3a7a517fd7ae47bc84a292904
Reviewed-on: https://go-review.googlesource.com/c/build/+/211317
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

app/appengine/ui.go

@@ -799,7 +799,6 @@ var tmplFuncs = template.FuncMap{
 	"shortDesc":          shortDesc,
 	"shortHash":          shortHash,
 	"shortUser":          shortUser,
-	"tail":               tail,
 	"unsupported":        unsupported,
 	"isUntested":         isUntested,
 	"formatTime":         formatTime,
@@ -913,15 +912,6 @@ func shortUser(user string) string {
 	return user
 }
 
-// tail returns the trailing n lines of s.
-func tail(n int, s string) string {
-	lines := strings.Split(s, "\n")
-	if len(lines) < n {
-		return s
-	}
-	return strings.Join(lines[len(lines)-n:], "\n")
-}
-
 // templateFile returns the path to the provided HTML template file,
 // conditionally prepending a relative path depending on the
 // environment.