bin/gen_release_notes: Remove cast that does nothing

Without an assignment this cast is dead code.

Cast's in python work like:
```python
from typing import cast
x = ['a', 'b', 'c']
x = cast(list[str | int], x)
x.append(5)
```

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38492>

bin/gen_release_notes.py

@@ -226,7 +226,6 @@ async def gather_bugs(version: str) -> typing.List[str]:
     loop = asyncio.get_event_loop()
     async with aiohttp.ClientSession(loop=loop) as session:
         results = await asyncio.gather(*[get_bug(session, i) for i in issues])
-    typing.cast(typing.Tuple[str, ...], results)
     bugs = list(results)
     if not bugs:
         bugs = ['None']