interfaces, gui: Remove is_mine output parameter from getAddress

The is_mine output parameter is never used by any callers.

src/qt/walletmodel.cpp

@@ -268,7 +268,7 @@ void WalletModel::sendCoins(WalletModelTransaction& transaction)
                 // Check if we have a new address or an updated label
                 std::string name;
                 if (!m_wallet->getAddress(
-                     dest, &name, /* is_mine= */ nullptr, /* purpose= */ nullptr))
+                     dest, &name, /*purpose=*/nullptr))
                 {
                     m_wallet->setAddressBook(dest, strLabel, wallet::AddressPurpose::SEND);
                 }