refactor(router): drop redundant ttft deadline guard

asyncio.wait_for already raises TimeoutError for a non-positive timeout, so
the explicit remaining<=0 check duplicated wait_for's own behavior. Removing
it keeps the same observable result (litellm.Timeout via the except clause)
and lets wait_for return an already-ready chunk instead of spuriously timing
out.

litellm/router.py

@@ -2917,8 +2917,6 @@ class Router:
                             chunk = await aiter.__anext__()
                         else:
                             remaining = deadline - loop.time()
-                            if remaining <= 0:
-                                raise asyncio.TimeoutError
                             chunk = await asyncio.wait_for(
                                 aiter.__anext__(), timeout=remaining
                             )