cmd/compile: remove -s flag

This is better handled by tools like cmd/gofmt, which can
automatically rewrite the source code and already supports a syntactic
version of this simplification. (go/types can be used if
type-sensitive simplification is actually necessary.)

Change-Id: I51332a8f3ff4ab3087bc6b43a491c6d92b717228
Reviewed-on: https://go-review.googlesource.com/c/go/+/197118Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -238,7 +238,6 @@ func Main(archInit func(*Arch)) {
 	if sys.RaceDetectorSupported(objabi.GOOS, objabi.GOARCH) {
 		flag.BoolVar(&flag_race, "race", false, "enable race detector")
 	}
-	objabi.Flagcount("s", "warn about composite literals that can be simplified", &Debug['s'])
 	if enableTrace {
 		flag.BoolVar(&trace, "t", false, "trace type-checking")
 	}

src/cmd/compile/internal/gc/typecheck.go

@@ -2747,11 +2747,6 @@ func pushtype(n *Node, t *types.Type) {
 		n.Right = typenod(t)
 		n.SetImplicit(true)       // don't print
 		n.Right.SetImplicit(true) // * is okay
-	} else if Debug['s'] != 0 {
-		n.Right = typecheck(n.Right, ctxType)
-		if n.Right.Type != nil && types.Identical(n.Right.Type, t) {
-			fmt.Printf("%v: redundant type: %v\n", n.Line(), t)
-		}
 	}
 }