cryptsetup: drop an unused variable

cryptsetup: drop an unused variable

This fixes compilation with new-enough libcryptsetup (2.4.0+) & clang:

```
$ CC=clang CXX=clang++ meson build --werror -Dlibcryptsetup-plugins=true
...
$ ninja -C build
...
../src/cryptsetup/cryptsetup-tokens/luks2-fido2.c:23:53: error: unused variable 'v' [-Werror,-Wunused-variable]
        _cleanup_(json_variant_unrefp) JsonVariant *v = NULL;
...
```