How to find file that placed a line in my header

Install the String Locator WordPress plugin (free):
https://wordpress.org/plugins/string-locator/

You can then search through all the files on your server, looking for google-analytics.com and do some inspection on the files found to see if any of those are the culprit. Chances are it’s hard coded in your theme header files, or there’s a plugin that’s enqueuing the script.

You can also use a DB search and replace plugin, just set config to do a “dry run” to search for google-analytics.com to see if it’s saved in the db anywhere:
https://wordpress.org/plugins/search-and-replace/

I would also recommend that you try using a different browser as well to check this, to make sure some Chrome (or whatever browser you’re using), that an extension is not injecting that on the page.