[client-python] Remove external reference from mapping cache

[helene-nguyen]

Proposed changes

• Remove external reference from mapping cache

Related issues

• External reference not (re)created when using bundle and OpenCTIStix2.put_attribute_in_extension opencti#7217

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...