NPUW: Fix memory clean-up for MoE model parts (-11GB) (#35910)
### Details:
- After the MoE refactoring (#35603 ) some parts of the transformed MoE
pieces remain in memory holding the tensors & references to the initial
.bin
### Tickets:
- EISW-216649
### AI Assistance:
- *AI assistance used: yes*
- *Autofix*
