fstab-generator: parse_new_root_from_proc_cmdline() don't use "defaults"

Don't use "defaults" as default option string.

"defaults,<anyotheroption>" does not
even work for the mount unit mount options.

src/fstab-generator/fstab-generator.c

@@ -449,7 +449,6 @@ static int parse_new_root_from_proc_cmdline(void) {
                 return 0;
         }
 
-        opts = strdup("defaults");
         type = strdup("auto");
         if (!opts || !type)
                 return log_oom();