fix: weight for DID deletion hook

[ntn-x2]

Fixes https://github.com/KILTprotocol/ticket/issues/3752.

I realized I over-complicated the previous solution. We never ever have considered the total size of a storage map in our benchmarks, so I don't see why we would need to start now.

In our case, the deletion hook has the worst case if it returns true, which is the default behaviour, hence we don't need any special setup logic for the benchmarks. If needed in the future, that is easy to add behind a feature = "runtime-benchmarks" cfg instruction.

This solution simplifies weight computation by a lot, since the additional weight will be considered when running the benchmarks already, and does not need any specialised benchmarking logic.

[Ad96el]