fix(studio): remove litellm dep (quarantined on PyPI) (#4553)

litellm has been quarantined on PyPI due to a supply chain attack
in version 1.82.8 (malicious credential-stealing .pth file).
No versions are currently installable, which blocks
`unsloth studio setup` at step 8/11 (data-designer deps).

Remove litellm from the single-env data-designer requirements
so setup completes. litellm can be re-added once PyPI lifts the
quarantine.

Ref: https://github.com/BerriAI/litellm/issues/24512

studio/backend/requirements/single-env/data-designer-deps.txt

@@ -8,7 +8,6 @@ httpx-retries<1,>=0.4.2
 json-repair<1,>=0.48.0
 jsonpath-rust-bindings<2,>=1.0
 jsonschema<5,>=4.0.0
-litellm<1.80.12,>=1.73.6
 lxml<7,>=6.0.2
 marko<3,>=2.1.2
 networkx<4,>=3.0