remove padding, should be managed by ThemeData

[sgruhier]

remove hardcoded padding to be able to set it up on ThemeData
example:

    ThemeData(
      buttonTheme: ButtonThemeData(
        padding: const EdgeInsets.only(left: 10, top: 20, bottom: 20),
        shape: RoundedRectangleBorder(
          side: const BorderSide(color: Colors.grey),
          borderRadius: BorderRadius.circular(20),
        ),
      ),
    )