Remove quantization/saving changes that conflict with PR keras-team#21713

- Remove variable_loading.py (quantization/saving related)
- Fix duplicate import in core_test.py
- Revert layer files to remove quantization changes
- Keep only core JAX memory management changes for OOM fix

Author: amitsrivastava78

keras/src/backend/jax/core_test.py

@@ -10,9 +10,6 @@
 from keras.src import testing
 from keras.src.backend.config import is_nnx_enabled
 
-if is_nnx_enabled():
-    from keras.src.backend.jax.core import NnxVariable
-
 if is_nnx_enabled():
     from flax import nnx