[SYCL] Prevent unnecessary command batching in L0 plug-in

[dm-vodopyanov]

This patch nullifies Queue->LastCommandEvent in Level Zero's
piQueueFinish as it was in _pi_event::cleanup to prevent extra batching
and performance drop.

[smaslov-intel]

/summary:run

[smaslov-intel]

LGT, but I requested a /summary:run

[bader]

@dm-vodopyanov, I see three different "short" description of the change and I'd like to chose the one to use for the commit title:

1. Nullify Queue->LastCommandEvent in L0's piQueueFinish (PR title)
2. Prevent extra batching and performance drop (from PR description)
3. Prevent unneeded already finished events to show up in the wait list (from code comments)

IMHO (1) might be clear only for a few people developing Level Zero plug-in.
Something like "Prevent unnecessary command batching in L0 plug-in" sounds more friendlier for the rest of us. :)
PR description already provides more detailed description for plug-in developers familiar with the code.
Please, update PR title if you agree with the proposal.

[dm-vodopyanov]

@bader, thanks for the suggestion! Changed, it looks much better.