Core bug due to due to Civi/Listeners.

When a change related to `Case` occurs Post hook gets called which inturn calls `fireCaseChange` function in `Civi\CCase` in which case updation takes place due to Civi/Listeners.
The same happens for deletion.

Therefore the delete api for `Case` ie `api_v3_CaseTest.testCaseDelete` used to break since the related `Case` is already deleted from the DB.
Hence skipping the call to update when operation is `delete`.