pallet-collator-selection: correctly register weight in `new_session` (#5430)

The `pallet-collator-selection` is not correctly using the weight for
the
[new_session](https://github.com/blockdeep/pallet-collator-staking/blob/main/src/benchmarking.rs#L350-L353)
function.

The first parameter is the removed candidates, and the second one the
original number of candidates before the removal, but both values are
swapped.