[megatron] fix: remove the demising critic.model.enable_gradient_checkpointing flags in the scripts (#2864)

### What does this PR do?

They were removed in #2651, but #2691 overlooked some of them.

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Signed-off-by: Hollow Man <hollowman@opensuse.org>

examples/ppo_trainer/run_deepseek_full_hh_rlhf.sh

@@ -23,7 +23,6 @@ python3 -m verl.trainer.main_ppo --config-path=./config --config-name='ppo_megat
     actor_rollout_ref.rollout.gpu_memory_utilization=0.4 \
     critic.optim.lr=1e-5 \
     critic.model.path=deepseek-ai/deepseek-llm-7b-chat \
-    critic.model.enable_gradient_checkpointing=False \
     critic.ppo_micro_batch_size_per_gpu=4 \
     reward_model.enable=True \
     reward_model.megatron.tensor_model_parallel_size=4 \

examples/ppo_trainer/run_qwen1.5_moe_a2.7b-gsm8k_megatron.sh

@@ -54,7 +54,6 @@ python3 -m verl.trainer.main_ppo --config-path=./config --config-name='ppo_megat
     actor_rollout_ref.rollout.tensor_model_parallel_size=$VLLM_TP \
     critic.optim.lr=1e-5 \
     critic.model.path=$HF_MODEL_PATH \
-    critic.model.enable_gradient_checkpointing=False \
     critic.ppo_micro_batch_size_per_gpu=4 \
     critic.megatron.tensor_model_parallel_size=$TP \
     critic.megatron.pipeline_model_parallel_size=$PP \