have a helper function `VaultReserve.clearVaultReserve()`

[chefburger]

          i wonder if we can have a helper function so its clearer eg. `VaultReserve.clearVaultReserve()` or something

and within the helper function, we can document why its set currencyLibrary.Native and 0 amount

Originally posted by @ChefMist in https://github.com/pancakeswap/pancake-v4-core/pull/165#discussion_r1749963883