Removes constraint in `BlockNumberProvider` from treasury (#6522)

https://github.com/paritytech/polkadot-sdk/pull/3970 updated the
treasury pallet to support relay chain block number provider. However,
it added a constraint to the BlockNumberProvider to have the same block
number type as frame_system:

```rust
type BlockNumberProvider: BlockNumberProvider<BlockNumber = BlockNumberFor<Self>>;
```

This PR removes that constraint as suggested by @gui1117