Remove cruft

* VDKTransactionViewController
    * Displays a transaction in a webview. @ayanonagon is it ok to remove this?
* VDKActivityView
    * used by VDKTransactionViewController
* `VenmoSDK webURLWithPath:`
    * unused method, not sure what it was for originally.
* VenmoSDK_Private
    * moved this class extension into the Spec file, which I think is cleaner.