logout static method
ログアウトする
Implementation
static Future<void> logout() async {
NCMBUser.ncmb!.sessionToken = null;
try {
NCMBUser._prefs = await SharedPreferences.getInstance();
NCMBUser._prefs!.remove(NCMBUser._userKey);
NCMBUser._prefs!.remove(NCMBUser._sessionKey);
} catch (e) {}
return;
}