If a test case is no more required or is considered to have been wrongly added then it can be deleted by a user with right to do so.
Open the test case that you want to delete.
Click the action menu button (located on the test case view's upper right corner).
What would happen when a test case is deleted?
The test case deletion is permanent and all its data would be removed from the database, along with its references from test plans it was part of and this includes the execution logs.
Quick tip: Use bulk actions to select multiple test cases at one time and delete them