remove ErrExist check on -debugdir's RemoveAll

This code initially used os.Stat, where it made sense to use
os.IsNotExist to catch whether the directory didn't exist.

However, we later transitioned to os.RemoveAll, which will never return
neither ErrExist nor ErrNotExist:

	If the path does not exist, RemoveAll returns nil (no error).

Simplify the code.

main.go

@@ -458,7 +458,7 @@ This command wraps "go %s". Below is its help:
 			flagDebugDir = filepath.Join(wd, flagDebugDir)
 		}
 
-		if err := os.RemoveAll(flagDebugDir); err == nil || errors.Is(err, fs.ErrExist) {
+		if err := os.RemoveAll(flagDebugDir); err == nil {
 			err := os.MkdirAll(flagDebugDir, 0o755)
 			if err != nil {
 				return nil, err