scanner: no longer dump debug messages to stderr

When varlink_object_new_from_json is fuzzed it receives a lot of bogus
json strings and clutters stderr with messages like "2 != 4" instead of
just returning errors. Given that it's the only place where fprintf is
used like that it seems it was left there accidentally and can
probably be safely removed to make the API more consistent in general.

It should make it easier to fuzz the library and other than that it
should also make it possible to compile libvarlink on some architectures
without compiler warnings like
```
../lib/scanner.c:628:49: warning: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Wformat]
                fprintf(stderr, "%ld != %ld\n", utf8_len, size);
                                 ~~~            ^~~~~~~~
                                 %zu
../lib/scanner.c:628:59: warning: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Wformat]
                fprintf(stderr, "%ld != %ld\n", utf8_len, size);
                                        ~~~               ^~~~
                                        %zu
```

lib/scanner.c

@@ -625,7 +625,6 @@ long scanner_expect_string(Scanner *scanner, char **stringp) {
         utf8_len = size;
         c_utf8_verify(&utf8_str, &utf8_len);
         if (utf8_len != 0) {
-                fprintf(stderr, "%ld != %ld\n", utf8_len, size);
                 scanner_error(scanner, SCANNER_ERROR_INVALID_CHARACTER);
                 return -VARLINK_ERROR_INVALID_JSON;
         }