[prep/test] have TxOrphanage remember its own limits in LimitOrphans

Move towards a model where TxOrphanage is initialized with limits that
it remembers throughout its lifetime.
Remove the param. Limiting by number of unique orphans will be removed
in a later commit.
Now that -maxorphantx is gone, this does not change the node behavior.
The parameter is only used in tests.

src/net_processing.h

@@ -76,8 +76,6 @@ public:
         bool ignore_incoming_txs{DEFAULT_BLOCKSONLY};
         //! Whether transaction reconciliation protocol is enabled
         bool reconcile_txs{DEFAULT_TXRECONCILIATION_ENABLE};
-        //! Maximum number of orphan transactions kept in memory
-        uint32_t max_orphan_txs{node::DEFAULT_MAX_ORPHAN_TRANSACTIONS};
         //! Number of non-mempool transactions to keep around for block reconstruction. Includes
         //! orphan, replaced, and rejected transactions.
         uint32_t max_extra_txs{DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN};