Remove https://github.com/kirillDanshin/nulltime

[invzhi]

https://github.com/kirillDanshin/nulltime

• [x] The project has not made an official release within the last year and has open issues.
• [ ] The project is not responding to bug reports issued within 6 months of submission.
• [ ] The project is not meeting quality standards as indicated by the Go Report Card or Code Coverage tests.
• [ ] The quality standard links have been removed from the documentation.
• [ ] The project is no longer open-sourced.
• [ ] The project is incompatible with any Go version issued within the last year (there is hopefully an open PR about this at the project).

This project is very old (7 years ago), and Go stdlib has been support sql.NullTime since Go 1.13.

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