Add removeIndex to SingleSlotMapper

[naddison36]

SameAssetUnderlyingsAbstractVault.removeVault currently calls SingleSlotMapper.removeValue with the removedValue parameter being the index of the _activeUnderlyingVaults array.

It would be more gas efficient if SameAssetUnderlyingsAbstractVault.removeVault called a new SingleSlotMapper.removeIndex function padding in the vaultIndex that is passed to the SameAssetUnderlyingsAbstractVault.removeVault function.