[in_app_purchase] remove `callbackChannel`

[callBackChannel](https://github.com/flutter/plugins/blob/master/packages/in_app_purchase/lib/src/channel.dart#L10) was added due to historic reasons. We shouldn't need it. We can use [channel](https://github.com/flutter/plugins/blob/master/packages/in_app_purchase/lib/src/channel.dart#L7) instead.

Note: this is a breaking change.