delete $color-slack in atom/custom-control

src/client/js/components/SlackNotification.jsx

@@ -80,7 +80,7 @@ SlackNotification.propTypes = {
   slackChannels: PropTypes.string.isRequired,
   onEnabledFlagChange: PropTypes.func,
   onChannelChange: PropTypes.func,
-  id: PropTypes.string,
+  id: PropTypes.string.isRequired,
 };
 
 export default withTranslation()(SlackNotification);

src/client/styles/scss/atoms/_custom_control.scss

@@ -71,8 +71,6 @@ label.custom-control-label {
 }
 
 .custom-switch.custom-switch-slack {
-  $color-slack: #4b144c;
-
   .custom-control-label {
     &::before {
       background-color: $gray-200;