Revert "nspawn: catch config mistake of specifying -b and args"

This reverts commit cb96a2c69a.

It is not a mistake to pass args when -b is specified. They will simply
be passed on to the container's init.

The manpage needs fixing, that's true.

src/nspawn/nspawn.c

@@ -267,11 +267,6 @@ static int parse_argv(int argc, char *argv[]) {
                 }
         }
 
-        if (optind < argc && arg_boot) {
-                log_error("Cannot specify a command together with '-b'");
-                return -EINVAL;
-        }
-
         return 1;
 }