Remove erroneous sideEffects in package.json

This unfortunately entirely breaks tree-shaking of the library, but
pending resolution of vuejs/rollup-plugin-vue#401 that's not something
we can fix on our end. This takes the approach that working-but-too-big
is at least better than not-working-at-all.

package.json

@@ -6,7 +6,6 @@
   "license": "MIT",
   "repository": "phosphor-icons/phosphor-vue",
   "homepage": "https://phosphoricons.com",
-  "sideEffects": false,
   "main": "dist/phosphor-vue.ssr.js",
   "browser": "dist/esm/entry.js",
   "module": "dist/esm/entry.js",