Delete metrics for all deleted queues in a single operation

Rather than calling `rabbit_core_metrics:delete_queue/1` for every
queue, collect all deleted queues and delete all their metrics in a
single operation.

We don't use a single Mnesia transaction to delete all objects related
to a queue and this may be a problem, but we haven't run this version of
the code long enough to know for sure. What should we be looking out for
@michaelklishin?

For initial context, see #1513

Partner-in-crime: @essen

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