Edit priority repetition

[nus-pe-bot]

tasks with priority that are already 0 can still be have their priorities deleted and set to 0 again

---

_{[original: nus-cs2113-AY2223S2/pe-interim#661] [original labels: severity.Low type.FeatureFlaw]}

[clement559]

Team's Response

We have already specified in the UG that certain commands with no effect can still be executed, but we may improve this in the future by displaying an error message instead.

Duplicate status (if any):

--