UI: remove useTrigger from TriggerDAGForm

[Lee-W]

Why

Address review feedback in apache#64211 while keeping materialize and trigger separate and avoiding callback/type confusion.

What

• Kept TriggerDAGModal.tsx unchanged.
• Simplified TriggerDAGForm.tsx to use a single callback prop: onSubmitTrigger.
• Removed unused onTrigger compatibility path after confirming no call sites use it.
• Preserved submit behavior: pause toggle (if checked) then onSubmitTrigger invocation.
• Keeps TypeScript clean without possibly undefined callback invocation.

Validation

• pnpm run lint (in airflow-core/src/airflow/ui)

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Was generative AI tooling used to co-author this PR?

• Yes — GPT-5 Codex

Generated-by: GPT-5 Codex following the guidelines