fix(notifications) remove unused isDismissAllowed prop

react/features/notifications/components/AbstractNotification.js

      */
     icon?: String,
 
-    /**
-     * Whether or not the dismiss button should be displayed.
-     */
-    isDismissAllowed: boolean,
-
     /**
      * Maximum lines of the description.
      */
      * @static
      */
     static defaultProps = {
-        appearance: NOTIFICATION_TYPE.NORMAL,
-        isDismissAllowed: true
+        appearance: NOTIFICATION_TYPE.NORMAL
     };
 
     /**

react/features/notifications/components/native/Notification.js

      * @returns {ReactElement}
      */
     render() {
-        const { isDismissAllowed } = this.props;
-
         return (
             <View
                 pointerEvents = 'box-none'
                         }
                     </View>
                 </View>
-                {
-                    isDismissAllowed
-                    && <TouchableOpacity onPress = { this._onDismissed }>
-                        <Icon
-                            src = { IconClose }
-                            style = { styles.dismissIcon } />
-                    </TouchableOpacity>
-                }
+                <TouchableOpacity onPress = { this._onDismissed }>
+                    <Icon
+                        src = { IconClose }
+                        style = { styles.dismissIcon } />
+                </TouchableOpacity>
             </View>
         );
     }