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?
Is the function_exists() necessary, now that we've moved the function?