submit_without_event impl still creates events for ooo queue

[JackAKirk]

Whilst submit_without_event on ooo queue doesn't return an event to the user, it still internally behaves the same as a normal submission due to this logic:

https://github.com/intel/llvm/blob/sycl/sycl/source/detail/queue_impl.cpp#L447

The extension specification doesn't point this out (although technically it isn't required since this is implementation detail) https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/experimental/sycl_ext_oneapi_enqueue_functions.asciidoc

But since this was requested here to remove this queue_impl property , supportsDiscardingPiEvents, here #18059 (comment)
I tried removing completely supportsDiscardingPiEvents but this leads to failures in standard situations of ooo queue use, since internally depends_on is used, which leads to the exception:

terminate called after throwing an instance of 'sycl::_V1::exception'
  what():  Queue operation cannot depend on discarded event.

I think it makes sense now ooo queue needs supportsDiscardingPiEvents currently, since for accessors at least it needs to be able to sync events, however it would probably be possible to optimize this out for ooo queues that don't ever use accessors. So this is a potential ooo queue optimization for the future.