Remove obsolete fetch_model_system_species.py script

This script was created in commit a54496c to fetch species data from external
APIs (Cellosaurus, Jackson Lab), but was never used in any workflow or other
script.

The sync_model_systems.py script now gets species data directly from the
NF Tools Database (syn51730943), making this script obsolete.

No functionality lost - species data is already being synced correctly.

Author: BelindaBGarana