Remove JAX reference-holding functionality and related tests

- Remove _ProtectedShardedArray class and _maybe_create_strong_reference method from core.py
- Remove jax.block_until_ready calls that are no longer needed
- Simplify variable initialization and assignment logic
- Remove all test cases related to reference holding from core_test.py
- Tests now pass and are consistent with the simplified implementation

Author: amitsrivastava78