The reason is you are using HostGator and enable the feature is Google Analytics Integration, by enable this one, HostGator will automatically add this snipped of code into your main website and all of it sub or add-on domain:
<script src="https://wordpress.stackexchange.com/google_analytics_auto.js"></script></head>
I my self also faced with this issue, my main website is http://nguyenhuutrong.com and I got the above line of code when viewing source, I tried to have one fresh WordPress in my add-on domain, but it still be there. After I tried to install wordpress on my local (by using XAMPP) then I found out the root cause is Google Analytics Integration of HostGator.
You just going to cPanel, choose the Google Analytics Integration, at the configuration you just choose to uninstall the Google Analytics (UA) for your domain and it will work.