[client] fix handling of external reference delete for delete operation (opencti #11958)

[JeremyCloarec]

Proposed changes

• fix handling of external reference delete for delete operation

Related issues

• opencti #11958

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[SouadHadjiat]

I wonder why "external-reference" is part of STIX_CORE_OBJECTS list instead of STIX_META_OBJECTS, while in the backend it's a stix meta object.