Move bug delete signal into core api

I'll need to check whether I added a 2nd event here previously. 

The question you raise here is basically a good one ->

In the past, we called 2 custom fucntions in this function:

issue_delete_validate
issue_delete_notify

At the moment, I'm treating EVENT_BUG_DELETE as equivalent of issue_delete_validate.

Now, given I have an upcoming patch that i've not got to yet that moved custom functions to plugin hooks, logic would say I would have needed to add a 2nd event.

In that scenario, I'd say that the signal should occur before the history event - on the basis that the bug delete event might want to alter the bug object - for example, edit data on the bug.

As an aside, the fact we log this to history and then delete the history in the within 10 lines of each other in the function, makes me wonder if we need to log it in the first place...