If you are looking for the option to improve your wordpress website performance I will definitely recommend to go through post Speed up your wordpress site to load fast. However, if you are only concerned to Remove query strings from static resources your are at right place.
All you need to do is login to your wordpress site » Go to dashboard » Appearance » Editor » Select Theme Functions (functions.php). This code is pretty safe and will not break anything but still it is a good idea to take backup of your functions.php before doing any modification. You can paste this code anywhere in your functions.php file but try add it at bottom.
Once done click on update file and go to GTMetrix to check score against Remove query strings from static resources. If you are using any caching plugin like W3 Total cache, do clear the cache before you attempt to check the score.
The above method will definitely give boost you score against Remove query strings from static resources but can not handle the external items that are still blocking you to reach 100%. Please read the second to get higher score.
Once done just check your score against Remove query strings from static resources. You will be surprised to see boost.
I have checked my website and it also suggest me to remove these kind of things. But I wonder does it really hurt the website speed or just a point that need to remove.
If this has still not fixed, please use the ftp account to remove the code you have added to your function.php and upload the file after removing the code, this will fix the issue.