gogio: remove fallback for missing GOOS=ios support in Go <= 1.15

Go 1.16 is already old, and the GOOS detection code fails for me on
Nix:

$ go run gioui.org/cmd/gogio -target ios ../gio-example/kitchen
gogio: go tool dist list failed: go tool dist: FAILED: not a Git repo; must put a VERSION file in $GOROOT

The error message points to a Nix packaging issue, but removing the
fallback is easier to fix.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gogio/iosbuild.go

@@ -423,16 +423,6 @@ func archiveIOS(tmpDir, target, frameworkRoot string, bi *buildInfo) error {
 	lipo := exec.Command("xcrun", "lipo", "-o", exe, "-create")
 	var builds errgroup.Group
 	tags := bi.tags
-	goos := "ios"
-	supportsIOS, err := supportsGOOS("ios")
-	if err != nil {
-		return err
-	}
-	if !supportsIOS {
-		// Go 1.15 and earlier target iOS with GOOS=darwin, tags=ios.
-		goos = "darwin"
-		tags = "ios " + tags
-	}
 	for _, a := range bi.archs {
 		clang, cflags, err := iosCompilerFor(target, a, bi.minsdk)
 		if err != nil {
@@ -452,7 +442,7 @@ func archiveIOS(tmpDir, target, frameworkRoot string, bi *buildInfo) error {
 		cflagsLine := strings.Join(cflags, " ")
 		cmd.Env = append(
 			os.Environ(),
-			"GOOS="+goos,
+			"GOOS=ios",
 			"GOARCH="+a,
 			"CGO_ENABLED=1",
 			"CC="+clang,
@@ -488,24 +478,6 @@ func archiveIOS(tmpDir, target, frameworkRoot string, bi *buildInfo) error {
 	return ioutil.WriteFile(moduleFile, []byte(module), 0644)
 }
 
-func supportsGOOS(wantGoos string) (bool, error) {
-	geese, err := runCmd(exec.Command("go", "tool", "dist", "list"))
-	if err != nil {
-		return false, err
-	}
-	for _, pair := range strings.Split(geese, "\n") {
-		s := strings.SplitN(pair, "/", 2)
-		if len(s) != 2 {
-			return false, fmt.Errorf("go tool dist list: invalid GOOS/GOARCH pair: %s", pair)
-		}
-		goos := s[0]
-		if goos == wantGoos {
-			return true, nil
-		}
-	}
-	return false, nil
-}
-
 func iosCompilerFor(target, arch string, minsdk int) (string, []string, error) {
 	var (
 		platformSDK string