[PE-D] Delete module does not fully delete the module's lessons

[nus-pe-bot]

After executing the delete module command multiple times until the module button disappears, one of the module's lessons will still be stored inside the module.

To reproduce:

1. Add a module
2. Add multiple lessons to the module
3. Add multiple tasks to the module
4. Delete the module

Expected outcome:
All the module's lessons should be deleted after the delete module command is executed.

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Labels: severity.High type.FunctionalityBug
original: successs404/ped#6