feat!: remove Minter from x/mint genesis state (#2217)

Closes celestiaorg/celestia-app#2067

Before this PR, genesis.json looked like:

```json
    "mint": {
      "minter": {
        "inflation_rate": "0.080000000000000000",
        "annual_provisions": "0.000000000000000000",
        "previous_block_time": null,
        "bond_denom": "utia"
      }
    },
```

After this PR, genesis.json should be:

```json
    "mint": {
      "bond_denom": "utia"
    },
```

app/default_overrides.go

@@ -100,7 +100,7 @@ type mintModule struct {
 // DefaultGenesis returns custom x/mint module genesis state.
 func (mintModule) DefaultGenesis(cdc codec.JSONCodec) json.RawMessage {
 	genState := minttypes.DefaultGenesisState()
-	genState.Minter.BondDenom = BondDenom
+	genState.BondDenom = BondDenom
 
 	return cdc.MustMarshalJSON(genState)
 }

proto/celestia/mint/v1/genesis.proto

@@ -1,13 +1,12 @@
 syntax = "proto3";
 package celestia.mint.v1;
 
-import "gogoproto/gogo.proto";
-import "celestia/mint/v1/mint.proto";
-
 option go_package = "github.com/celestiaorg/celestia-app/x/mint/types";
 
 // GenesisState defines the mint module's genesis state.
 message GenesisState {
-  // minter is a space for holding current inflation information.
-  Minter minter = 1 [ (gogoproto.nullable) = false ];
+  reserved 1; // 1 was previously used for the `Minter` field.
+
+  // BondDenom is the denomination of the token that should be minted.
+  string bond_denom = 2;
 }

x/mint/keeper/genesis.go

@@ -7,7 +7,9 @@ import (
 
 // InitGenesis initializes the x/mint store with data from the genesis state.
 func (k Keeper) InitGenesis(ctx sdk.Context, ak types.AccountKeeper, data *types.GenesisState) {
-	k.SetMinter(ctx, data.Minter)
+	minter := types.DefaultMinter()
+	minter.BondDenom = data.BondDenom
+	k.SetMinter(ctx, minter)
 	// override the genesis time with the actual genesis time supplied in `InitChain`
 	blockTime := ctx.BlockTime()
 	gt := types.GenesisTime{
@@ -22,6 +24,6 @@ func (k Keeper) InitGenesis(ctx sdk.Context, ak types.AccountKeeper, data *types
 
 // ExportGenesis returns a x/mint GenesisState for the given context.
 func (k Keeper) ExportGenesis(ctx sdk.Context) *types.GenesisState {
-	minter := k.GetMinter(ctx)
-	return types.NewGenesisState(minter)
+	bondDenom := k.GetMinter(ctx).BondDenom
+	return types.NewGenesisState(bondDenom)
 }

x/mint/types/genesis.go

@@ -1,21 +1,26 @@
 package types
 
+import "errors"
+
 // NewGenesisState creates a new GenesisState object
-func NewGenesisState(minter Minter) *GenesisState {
+func NewGenesisState(bondDenom string) *GenesisState {
 	return &GenesisState{
-		Minter: minter,
+		BondDenom: bondDenom,
 	}
 }
 
 // DefaultGenesisState creates a default GenesisState object
 func DefaultGenesisState() *GenesisState {
 	return &GenesisState{
-		Minter: DefaultMinter(),
+		BondDenom: DefaultBondDenom,
 	}
 }
 
 // ValidateGenesis validates the provided genesis state to ensure the
 // expected invariants holds.
 func ValidateGenesis(data GenesisState) error {
-	return data.Minter.Validate()
+	if data.BondDenom == "" {
+		return errors.New("bond denom cannot be empty")
+	}
+	return nil
 }