Remove getTasks from Grid view

[bbovenzi]

We were only using GET dags/{dag_id}/tasks for a few things in the grid view and there were two issues:

• the request sometimes took multiple seconds for large dags
• we weren't directly retrieving a task's operator, instead we relied on checking task.class_ref.class_name

By adding the operator info into grid_data, we can clean up a lot of code.

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