message: [Model] block zpiv mints from the ui.
diff: @@ -488,7 +488,7 @@ bool WalletModel::createZpivSpend(
  receipt,
  vMintsSelected,
  vNewMints,
  - fMintChange,
  + false, // No more mints
  fMinimizeChange,
  outputs,
  changeAdd
  @@ -523,7 +523,7 @@ bool WalletModel::sendZpiv(
  wtxNew,
  receipt,
  vMintsSelected,
  - fMintChange,
  + false, // No more mints
  fMinimizeChange,
  outputs,
  changeAdd
  @@ -553,7 +553,7 @@ bool WalletModel::convertBackZpiv(
  wtxNew,
  receipt,
  vMintsSelected,
  - fMintChange,
  + false, // No more mints
  fMinimizeChange,
  std::list<std::pair<CBitcoinAddress*, CAmount>>(),
  &addressTo
  