Remove mem-pool.total-bytes and hash-table.total-bytes metrics

I think we should remove these metrics - I'm not aware of a use case for them and I haven't seen them be of use since I've been working on Impala's memory management. Generally memory consumption is only interesting when broken down by query.

The hash-table metric is also wrong since it doesn't include the actual hash table buckets.

The description of the mem-pool metric is totally wrong - "The current size of the memory pool shared by all queries".