add AfterNftUnlisted hook and description of it

x/ecosystem-incentive/spec/04_hooks.md

@@ -10,6 +10,7 @@ The example hooks functions interfaces in x/nftmarket module:
 type NftmarketHooks interface {
    AfterNftListed(ctx sdk.Context, nft_id types.NftIdentifier, incentive_id string)
    AfterNftPaid(ctx sdk.Context, nft_id types.NftIdentifier, fee_amount mathInt, fee_denom string)
+   AfterNftUnlisted(ctx sdk.Context, nft_id types.NftIdentifier)
 }
 ```
 
@@ -22,3 +23,8 @@ To pass the `incentive_id` from the memo data of `MsgListNft` requires a method
 
 This hooks function is called for the accumulation of the reward for the subjects which are connected with the `nft_id` in the argument.
 The calculation of the actual reward amount is executed in methods which this hook function calls in this module.
+
+## AfterNftUnlisted
+
+This hook function is called when a nft is unlisted for some reason like liquidation.   
+The purpose is to remove the unlisted nft information from `IncentiveIdTable` KVStore to keep the data consystent.