refactor: simplify system variable access in TriggerScheduleNode

[google-labs-jules]

Simplified system variable access in TriggerScheduleNode by removing the manual extraction and injection of system variables into the node's output. System variables are already available via the VariablePool, making this step redundant.

Changes:

• Modified api/core/workflow/nodes/trigger_schedule/trigger_schedule_node.py to remove the loop that added system variables to outputs.
• Removed unused import SYSTEM_VARIABLE_NODE_ID.

Verified with a new unit test (temporarily created and run) ensuring system variables are no longer in outputs but user inputs are preserved.

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PR created automatically by Jules for task 3325533844711015309 started by @asukaminato0721

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