Refactor `native_account::remove_asset` to return the remaining balance

[PhilippGackstatter]

As motivated in #2523, we should avoid returning outputs identical to inputs in kernel procedures. After #2437, native_account::remove_asset currently takes and returns ASSET_VALUE. Instead of removing the return value, we can make it more useful by making it analogous to native_account::add_asset. It returns the asset after it was merged into the account's vault, which is useful to check the balance after addition without an additional kernel call. Similarly, remove_asset should return the asset remaining in the vault after the provided value was removed.

So this could be:

#! Inputs:  [ASSET_KEY, ASSET_VALUE]
#! Outputs: [REMAINING_ASSET_VALUE]