Remove relative_path when creating relPath

`req._parsedUrl` does not contain `relative_path` so don't include `relative_path` when removing the prefix.

For example, I installed nodeBB in `/forum' and the value of req._parsedUrl is

```javascript
{ protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: null,
  query: null,
  pathname: '/plugins/nodebb-plugin-emoji-extended/images/+1.png',
  path: '/plugins/nodebb-plugin-emoji-extended/images/+1.png',
  href: '/plugins/nodebb-plugin-emoji-extended/images/+1.png' }
```