shared: drop redundant dlopen_cryptsetup() calls from cryptsetup_* helpers

shared: drop redundant dlopen_cryptsetup() calls from cryptsetup_* helpers

cryptsetup_set_minimal_pbkdf(), cryptsetup_get_token_as_json() and
cryptsetup_add_token_json() each take a struct crypt_device *cd, which
can only be obtained by first calling sym_crypt_init*() — and that
already requires dlopen_cryptsetup() to have succeeded. The internal
calls here were only implicitly re-loading a library the caller is
guaranteed to have already loaded.