[Misc] Remove redundant statements in scheduler.py (vllm-project#13229)

Signed-off-by: Louis Ulmer <ulmerlouis@gmail.com>

Author: WrRan

vllm/core/scheduler.py

@@ -770,7 +770,6 @@ def _schedule_swapped(
             swapped_queue.popleft()
             self._swap_in(seq_group, blocks_to_swap_in)
             self._append_slots(seq_group, blocks_to_copy, enable_chunking)
-            is_prefill = seq_group.is_prefill()
             if is_prefill:
                 prefill_seq_groups.append(
                     ScheduledSequenceGroup(