fix build error

demo/collision.html

@@ -5,8 +5,6 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Shapes - Collision</title>
     <style>
-      @import url('../src/elements/main.css');
-
       * {
         box-sizing: border-box;
       }