remove unload warning. (#1704)

## Description:

The unload alert is triggered when purchasing items causing bad ux. We
already have unload alert in-game if you are still alive so this seems
unnecessary.

reverts:
https://github.com/openfrontio/OpenFrontIO/commit/ac8a841e40d0e9d290877b1fe71a0f9456758fa5

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I have read and accepted the CLA agreement (only required once).

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/client/index.html

@@ -442,10 +442,6 @@
           document.documentElement.classList.remove("preload");
         });
       });
-      window.addEventListener("beforeunload", function (e) {
-        e.preventDefault();
-        e.returnValue = "Are you sure you want to leave?";
-      });
     </script>
 
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