chore(pt): Delete deepmd/pt/utils/cache.py (#4356)

It's not used and not covered. Delete per discussion.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Removed a custom LRU cache decorator to streamline functionality and
reduce complexity.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>