Remove Python 3.9+ exclusive code from WebHost options.py.

`str.removeprefix` was added in Python 3.9, so this causes WebHost to return a 500 error if running on Python 3.8.

WebHostLib/options.py

@@ -234,7 +234,7 @@ def generate_yaml(game: str):
         # Detect random-* keys and set their options accordingly
         for key, val in options.copy().items():
             if key.startswith("random-"):
-                options[key.removeprefix("random-")] = "random"
+                options[key[len("random-"):]] = "random"
                 del options[key]
 
         # Error checking