Add missing API to remove a single user script

Number:rdar://FB7557869 Date Originated:
Status:Open Resolved:
Product:WebKit Product Version:
Classification:Suggestion Reproducible:
Currently when dealing with injecting user scripts using WKUserScript into a WKWebView you only have the option to remove all the user scripts using `WKUserContentController.removeAllUserScripts()`. 

It would be nice to be able to remove only a specific user script that has been added instead of having to remove all of them and then re-add the user scripts minus the one removed. At Brave, we must do this in order to remove user scripts based on the page you are visiting, since we don't want to have certain user scripts injected for all pages.


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