server: Remove superfluous check in OnMemPool. (#736)
server: Remove superfluous check in OnMemPool. (#736)

This reduces the mempool lock contention by removing an unnecessary
check when responding to a "mempool" request.

In particular, the code first gets a list of all transactions from the
mempool and then iterates them in order to construct the inventory
vectors and apply bloom filtering if it is enabled.  Since it is
possible that the transaction was removed from the mempool by another
thread while that list is being iterated, the code was checking if each
transaction was still in the mempool.  This is a pointless check because
the transaction might still be removed at any point after the check
anyways.  For example, it might be removed after the mempool response
has been sent to the remote peer or even while the loop is still
iterating.
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