Use explicit jQuery import, remove unused eslint globals (#18435)

- Don't rely on globals (window.$) for jQuery import
- Remove eslint globals no longer in use

web_src/js/features/heatmap.js

@@ -1,5 +1,4 @@
 import Vue from 'vue';
-
 import ActivityHeatmap from '../components/ActivityHeatmap.vue';
 
 export default function initHeatmap() {