In addition to the 20% of the script writing you did on the clock, you used your job as the test environment.
If you regard your scripts as your property, not the company's, did you check their policies on the use of outside software? How was the software licensed? Did your manager agree to its use? A license that says, in effect, "You can use these scripts until Matt Damoz leaves." would generally not be acceptable, where one of the standard open source licenses would have been fine.
Your main responsibility during your notice period will be to document your work and hand it over. If you delete scripts you were using, and try to revert during the notice period to an earlier way of doing the jobs, that will be much, much harder to do well. How clearly do you remember, and can you describe, how you used to do things before writing the first script?
The main sufferers if you delete the scripts will be whoever has to do your work. Even if you are on good terms with them now, you won't be after they have struggled with the consequences of the script deletion. They will attribute their difficulties to either malice or incompetence on your part, affecting how they will talk about you, and how they will interact with you if you end up as colleagues in the future.
There has been some discussion in comments on the ownership of the code. That is a legal issue, related to employment and intellectual property law in the jurisdiction of the OP's employment. It appears to me that there would be a serious professionalism issue either in using or in deleting the code in all cases.
The possibilities divide into three major cases:
- The OP developed the code for the employer, and it belongs to the employer. Any code written on the OP's own time was donated to the employer.
- The code belongs to the OP.
- The code is joint property of the OP and the employer, resulting from a collaboration between them.
Case 1, an employee as employee writing scripts to make their work more efficient, is OK so far, but would make deleting the scripts very unprofessional, if not something for which the OP could be sued.
Cases 2 and 3 are more complicated. In each case, the OP as employee is using code that does not belong outright to the employer. It would be very unprofessional for the OP to do so without following procedures for obtaining a license and/or contract with the OP as independent developer. The OP could not properly reach that sort of agreement on behalf of the employer, even if otherwise authorized to do so, because of the conflict of interest involved.
The OP's hands would be clean if the OP did indeed follow corporate procedures for using outside code and avoiding conflicts of interest. However, the employer would probably have required a license that permits continued use of the code after the OP leaves.