1. Remove some redundant code.
2. Fix the issue with the state of MTMV task.
3. Fix the case - test_create_mtmv.
## Problem summary
1. We used a retry policy to re-run the failed MTMV tasks, but we set the state to `FAILURE` during re-running the tasks.
We should do this after all the retry runs fail.
2. There are some redundant code can be removed.
3. In the case test_create_mtmv, we created many background tasks to refresh the data. Some task may fail due to the concurrency and cause the test fail. Actually, we only need single one task to verify the functionality.
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