qt export history: let util.filename_field decide default path

which uses:
directory = config.get('io_dir', os.path.expanduser('~'))

electrum/gui/qt/history_list.py

@@ -826,7 +826,7 @@ class HistoryList(MyTreeView, AcceptFileDragDrop):
         d = WindowModalDialog(self, _('Export History'))
         d.setMinimumSize(400, 200)
         vbox = QVBoxLayout(d)
-        defaultname = os.path.expanduser('~/electrum-history.csv')
+        defaultname = f'electrum-history.csv'
         select_msg = _('Select file to export your wallet transactions to')
         hbox, filename_e, csv_button = filename_field(self, self.config, defaultname, select_msg)
         vbox.addLayout(hbox)