index: drop react-complex-tree css

since we are using a custom renderer, we are not using any of this css.

src/index.scss

@@ -5,7 +5,6 @@
 @import '~normalize.css';
 @import '~@blueprintjs/core/src/blueprint.scss';
 @import '~@blueprintjs/popover2/src/blueprint-popover2.scss';
-@import '~react-complex-tree/lib/style.css';
 
 :root {
     --pb-vh: 100vh;