feat!: remove legacy REST (#9594)

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## Description

ref: #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 #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 #9615
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

server/api/server.go

@@ -15,10 +15,10 @@ import (
 	tmrpcserver "github.com/tendermint/tendermint/rpc/jsonrpc/server"
 
 	"github.com/cosmos/cosmos-sdk/client"
+	"github.com/cosmos/cosmos-sdk/codec/legacy"
 	"github.com/cosmos/cosmos-sdk/server/config"
 	"github.com/cosmos/cosmos-sdk/telemetry"
 	grpctypes "github.com/cosmos/cosmos-sdk/types/grpc"
-	"github.com/cosmos/cosmos-sdk/types/rest"
 
 	// unnamed import of statik for swagger UI support
 	_ "github.com/cosmos/cosmos-sdk/client/docs/statik"
@@ -133,7 +133,7 @@ func (s *Server) registerMetrics() {
 
 		gr, err := s.metrics.Gather(format)
 		if err != nil {
-			rest.WriteErrorResponse(w, http.StatusBadRequest, fmt.Sprintf("failed to gather metrics: %s", err))
+			writeErrorResponse(w, http.StatusBadRequest, fmt.Sprintf("failed to gather metrics: %s", err))
 			return
 		}
 
@@ -143,3 +143,22 @@ func (s *Server) registerMetrics() {
 
 	s.Router.HandleFunc("/metrics", metricsHandler).Methods("GET")
 }
+
+// errorResponse defines the attributes of a JSON error response.
+type errorResponse struct {
+	Code  int    `json:"code,omitempty"`
+	Error string `json:"error"`
+}
+
+// newErrorResponse creates a new errorResponse instance.
+func newErrorResponse(code int, err string) errorResponse {
+	return errorResponse{Code: code, Error: err}
+}
+
+// writeErrorResponse prepares and writes a HTTP error
+// given a status code and an error message.
+func writeErrorResponse(w http.ResponseWriter, status int, err string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_, _ = w.Write(legacy.Cdc.MustMarshalJSON(newErrorResponse(0, err)))
+}