cli: remove unused variable assigment

cli: remove unused variable assigment

scan-build from clang version 9 has reported following issue:

 cli.c:574:8: warning: Although the value stored to 'ret' is used in the enclosing expression, the value is never actually read from 'ret'
                if ((ret = uci_parse_argument(ctx, input, &str, &argv[i])) != UCI_OK) {
                     ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Petr Štetiar <ynezz@true.cz>