You can delete the test case which is no longer required. After you delete the test case, all the tasks from this test case are detached. You can access these tasks from the Assets > Tasks section.
...
Click the desired project name under which the test case exists.
From the top menu items, click the Test Cases menu.
Apply Filters to find the test case you want to delete. For more details, refer to Searching Test Cases.
In the Test Cases table, you can delete in the following two ways:
Directly Deleting a Test Case:
From the list of test cases, click the delete icon of the test case you want to delete.
A confirmation dialog appears displaying the impact of deleting the test case on coverage, if any.
Click Yes to delete the test case
By Opening a Test Case:
Open the test case that you want to delete.
Click the three dots menu options ( next to the Share for Automation button, and select Delete.
A confirmation dialog appears displaying the impact of deleting the test case on coverage, if any.
Click Yes to delete the test case.
...