[Structured Output] Remove redundant check for grammar_bitmask (#1459)

### What this PR does / why we need it?
Remove redundant check since we have check this at
https://github.com/vllm-project/vllm-ascend/blob/main/vllm_ascend/worker/model_runner_v1.py#L1450.


Signed-off-by: shen-shanshan <467638484@qq.com>

vllm_ascend/worker/model_runner_v1.py

@@ -1273,8 +1273,6 @@ class NPUModelRunner(LoRAModelRunnerMixin):
         # Serialization of np.ndarray is much more efficient than a tensor,
         # so we receive it in that format.
         grammar_bitmask = scheduler_output.grammar_bitmask
-        if grammar_bitmask is None:
-            return
 
         # We receive the structured output bitmask from the scheduler, but the
         # indices of the requests in the batch may not match the indices of