One would be able to delete a test case using a web UI. However if you want to automate using a script then the following call is probably you need to delete a test case. Here are the test case to delete a test case in GC stream.

• Find a test case URL. Example https://<host>:<port>/qm/oslc_qm/contexts/_aWIGcJzgEemulY4ghI7JGA/resources/com.ibm.rqm.planning.VersionedTestCase/_q88WgcgIEemXA_b2SztZnA/_KSKwJsgNEemXA_b2SztZnA (URL can be obtained using web UI Copy as an OSLC versioned URL)
• Open REST Client
• Place the URL but remove state id as this does not require to delete a test case. Example https://<host>:<port>/qm/oslc_qm/contexts/_aWIGcJzgEemulY4ghI7JGA/resources/com.ibm.rqm.planning.VersionedTestCase/_q88WgcgIEemXA_b2SztZnA/
• Set headers  for OSCL v2 (OSLC-Core-Version - 2.0)
• Pick up DELETE from RESTClient for action to perform.
• Hit send. You'll see Status Code: 200 OK if all goes well.