Remove notifications system

[RichDom2185]

Description

Reverts #2494 as per offline discussion.

Closes #2519.

To close subsequently: https://github.com/source-academy/backend/pull/950

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Code quality improvements

Checklist

• [x] I have tested this code
• [ ] I have updated the documentation

[coveralls]

Pull Request Test Coverage Report for Build 10340042876

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 31.736%

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Totals
Change from base Build 10338639552:	0.5%
Covered Lines:	4812
Relevant Lines:	14335

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💛 - Coveralls