refactor: Remove redundant PSBT copy constructor

The default (i.e., generated by a compiler) copy constructor does the 
same things.

Also this prevents -Wdeprecated-copy warning for implicitly declared 
operator= in GCC 9.

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