fix: remove empty environment key causing nemo_run ListParseError

nemo_run's CLI parser fails when `environment:` is present but has no
value (parsed as None, not a valid List[Dict[str, str]]).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenhan Yu <chenhany@nvidia.com>

Author: ChenhanYu

tools/launcher/examples/Qwen/Qwen3-8B/hf_online_dflash.yaml

@@ -36,7 +36,6 @@ pipeline:
       - dflash.dflash_loss_decay_factor=7
       - dflash.dflash_mask_token_id=151669
       - dflash.dflash_architecture_config.num_hidden_layers=5
-    environment:
     slurm_config:
       _factory_: "slurm_factory"
       nodes: 1