Remove cache from last synced block

Fix for: block_explorer_ui!455 (merged)

Removing caching the response in this particular endpoint results in minimalized delay in Explorer synced with blockchain (hafbe's last synced block is at most 1 block behind blockchain headblock instead of 1-3 blocks in some cases), but not completly fixes it, it most likely is related to hafbe's sync that takes around 1s per block.