jwt: remove jwt_payload_len function

The function jwt_payload_len doesn't return payload length but returns
used data in builder->buf

If jwt_payload_len is called after jwt_init_builder, header length will
be returned
If jwt_payload_len is called after jwt_add_payload, header+payload
length will be returned
If jwt_payload_len is called after jwt_sign, header+payload+sign length
will be returned

So, this commit removes the function and uses strlen instead

Signed-off-by: Benjamin Lemouzy <blemouzy@centralp.fr>