Fix treasury benchmarks when no SpendOrigin (#3049)

### Issue

It was impossible to benchmark the pallet_treasury when `SpendOrigin =
frame_support::traits::NeverEnsureOrigin<u64>;` was specified.

### Done

- [x] Use `weight = 0` for all extrinsics that are un-callable with no
`SpendOrigin`.
- [x] Fix benchmarks for extrinsics requiring a Spend even if
`SpendOrigin = frame_support::traits::NeverEnsureOrigin<u64>;`

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>