pallet nis: remove benchmarking hack (#13759)

* pallet nis: remove benchmark hack

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* NearestPrefDown -> Down

Tested on the kitchensink and in case that
n = 10000000000000000 and d = 12000100600000000000000 it rounds the
Perquintill up (correctly) since 833326347280+47318160/60000503 should round up.
That triggers the path 'amount <= on_hold' in L736.

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>