Revert "Defer processing of expansion interface interrupts (fixes audio ...

Reverts dolphin-emu/dolphin#1417

I already explained in the PR that this was not a threading issue, so the threadsafe thing is useless overhead.  It should not have been merged.  Furthermore, using any kind of event inside UpdateInterrupts is rather redundant when one of the two callers (IIRC) already uses one.  I suggest not making a change like this, but rather figuring out what the actual problem is and fixing it rather than blindly putting it off until "later".