remove currency from JSON API

[hohwille]

As a leftover of issue #48 we still have the currency of monetary amounts in the JSON API

"price": {"currency":"EUR","amount":"6.99"}

however the currency is now static and configured by a constant as this makes sense for a restaurant and follows KISS. Therefore the currency is ignored when JSON is send from the client.
The OASP4JS team should add a static configuration constant for the currency in their client as well and then we can change the JSON API such that the currency is removed:

"price": "6.99"

[agreul]

done