Revert "FIX: Auth popup handling for Safari same-site cookie quirks"

This reverts commit 310a8ac2427978779c1bb31f71321bfb84688336.

It seems this breaks google authentication. My suspicion is opening
the URL twice invalidates the CSRF after the first access.

app/assets/javascripts/discourse/models/login-method.js.es6

@@ -53,7 +53,7 @@ const LoginMethod = Ember.Object.extend({
         }
         LoginMethod.buildPostForm(authUrl).then(form => {
           const windowState = window.open(
-            authUrl,
+            "about:blank",
             "auth_popup",
             `menubar=no,status=no,height=${height},width=${width},left=${left},top=${top}`
           );