package.json: Uninstall 'string' package

We no longer use string@3.3.3 in our codebase.

string@3.3.3 is reported to be unsafe and contains vulnerabilities [1].

Let's remove string@3.3.3 as one of our dependencies.

[1] - jprichardson/string.js#212

package.json

@@ -21,7 +21,6 @@
     "markdown-it-mark": "^2.0.0",
     "markdown-it-sub": "^1.0.0",
     "markdown-it-sup": "^1.0.0",
-    "string": "^3.3.3",
     "vue": "^2.0.0"
   },
   "files": [