WPSC_Purchase_Log::delete() should delete related cart items, form data and claimed stock

For issue #1601

WPSC_Purchase_Log::delete() should delete other data associated with a purchase log.

This PR replicates the function of wpsc_delete_purchlog() and deprecates that function in favour of the class method.

My thoughts are the WPSC_Purchase_Log::delete() should really not be a static method (it does't actually appear to be used in core) but that might not be a possibility for back-compatibility.

@garyc40, The method seems to have been last updated by you 2 years ago. I know a lot of refactoring was going on at the time so I'm not sure if you have any further knowledge of wether this method was ever used?
No. Now that's in I'll remove it.