Merge #17349: Remove redundant copy constructors

fa8919889f bench: Remove redundant copy constructor in mempool_stress (MarcoFalke)
29f8434368 refactor: Remove redundant PSBT copy constructor (Hennadii Stepanov)

Pull request description:

  I fail to see why people add these copy constructors manually without explanation, when the compiler can generate them at least as good automatically with less code.

ACKs for top commit:
  promag:
    ACK fa8919889f.
  hebasto:
    ACK fa8919889f, nit s/constructor/operator/ in commit fa8919889f message, as @promag [mentioned](https://github.com/bitcoin/bitcoin/pull/17349#discussion_r341776389) above.
  jonatack:
    ACK fa8919889f

Tree-SHA512: ce024fdb894328f41037420b881169b8b1b48c87fbae5f432edf371a35c82e77e21468ef97cda6f54d34f1cf9bb010235d62904bb0669793457ed1c3b2a89723

src/bench/mempool_stress.cpp

@@ -23,12 +23,6 @@ struct Available {
     size_t vin_left{0};
     size_t tx_count;
     Available(CTransactionRef& ref, size_t tx_count) : ref(ref), tx_count(tx_count){}
-    Available& operator=(Available other) {
-        ref = other.ref;
-        vin_left = other.vin_left;
-        tx_count = other.tx_count;
-        return *this;
-    }
 };
 
 static void ComplexMemPool(benchmark::State& state)
@@ -66,7 +60,7 @@ static void ComplexMemPool(benchmark::State& state)
                 tx.vin.back().scriptSig = CScript() << coin.tx_count;
                 tx.vin.back().scriptWitness.stack.push_back(CScriptNum(coin.tx_count).getvch());
             }
-            if  (coin.vin_left == coin.ref->vin.size()) {
+            if (coin.vin_left == coin.ref->vin.size()) {
                 coin = available_coins.back();
                 available_coins.pop_back();
             }

src/psbt.h

@@ -401,7 +401,6 @@ struct PartiallySignedTransaction
     bool AddInput(const CTxIn& txin, PSBTInput& psbtin);
     bool AddOutput(const CTxOut& txout, const PSBTOutput& psbtout);
     PartiallySignedTransaction() {}
-    PartiallySignedTransaction(const PartiallySignedTransaction& psbt_in) : tx(psbt_in.tx), inputs(psbt_in.inputs), outputs(psbt_in.outputs), unknown(psbt_in.unknown) {}
     explicit PartiallySignedTransaction(const CMutableTransaction& tx);
     /**
      * Finds the UTXO for a given input index