Thanks for your posting. Deleting test cases also deletes all tests and results in active test runs so please make sure to close any finished, but active test runs you want to keep:
How to delete/deprecate test cases depends on your preference. You can physically delete them as you mentioned but this would remove their case history. Tests & results would not be affected though if you closed the runs & plans previously as mentioned.
If you want to keep the case history, it can make sense to deprecate the cases instead. Many customers use a custom field for this (e.g. Status dropdown or Obsolete checkbox), and you can use this field to configure your case selection when adding new test runs or plans.
I hope this helps!