feat!: remove legacy REST (cosmos#9594)

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## Description

ref: cosmos#7517 

  * [x] Remove the x/{module}/client/rest folder
  * [x] Remove all glue code between simapp/modules and the REST server

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---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification - see cosmos#9615
- [x] reviewed "Files changed" and left comments if necessary
- [x] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [x] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [x] reviewed API design and naming
- [ ] reviewed documentation is accurate - see cosmos#9615
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

app.go

@@ -19,7 +19,6 @@ import (
 	"github.com/cosmos/cosmos-sdk/baseapp"
 	"github.com/cosmos/cosmos-sdk/client"
 	"github.com/cosmos/cosmos-sdk/client/grpc/tmservice"
-	"github.com/cosmos/cosmos-sdk/client/rpc"
 	"github.com/cosmos/cosmos-sdk/codec"
 	"github.com/cosmos/cosmos-sdk/codec/types"
 	"github.com/cosmos/cosmos-sdk/server/api"
@@ -32,7 +31,6 @@ import (
 	"github.com/cosmos/cosmos-sdk/version"
 	"github.com/cosmos/cosmos-sdk/x/auth"
 	"github.com/cosmos/cosmos-sdk/x/auth/ante"
-	authrest "github.com/cosmos/cosmos-sdk/x/auth/client/rest"
 	authkeeper "github.com/cosmos/cosmos-sdk/x/auth/keeper"
 	authsims "github.com/cosmos/cosmos-sdk/x/auth/simulation"
 	authtx "github.com/cosmos/cosmos-sdk/x/auth/tx"
@@ -513,9 +511,6 @@ func (app *SimApp) SimulationManager() *module.SimulationManager {
 // API server.
 func (app *SimApp) RegisterAPIRoutes(apiSvr *api.Server, apiConfig config.APIConfig) {
 	clientCtx := apiSvr.ClientCtx
-	rpc.RegisterRoutes(clientCtx, apiSvr.Router)
-	// Register legacy tx routes.
-	authrest.RegisterTxRoutes(clientCtx, apiSvr.Router)
 	// Register new tx routes from grpc-gateway.
 	authtx.RegisterGRPCGatewayRoutes(clientCtx, apiSvr.GRPCGatewayRouter)
 	// Register new tendermint queries routes from grpc-gateway.