MM-41184: Remove /config/migrate API endpoint (#19418)

This was introduced to let the functionality happen
via mmctl, but it does not differentiate between
sysadmins vs people having physical access to machines.

Therefore, we remove it and only keep the local endpoint
accessible from mmctl --local.

https://mattermost.atlassian.net/browse/MM-41184

```release-note
The api/v4/config/migrate API endpoint has been removed
in favor of the mmctl --local endpoint. API clients
won't be able to access this endpoint without having
physical access to the server.
```

api4/config.go

@@ -35,7 +35,6 @@ func (api *API) InitConfig() {
 	api.BaseRoutes.APIRoot.Handle("/config/reload", api.APISessionRequired(configReload)).Methods("POST")
 	api.BaseRoutes.APIRoot.Handle("/config/client", api.APIHandler(getClientConfig)).Methods("GET")
 	api.BaseRoutes.APIRoot.Handle("/config/environment", api.APISessionRequired(getEnvironmentConfig)).Methods("GET")
-	api.BaseRoutes.APIRoot.Handle("/config/migrate", api.APISessionRequired(migrateConfig)).Methods("POST")
 }
 
 func init() {
@@ -385,40 +384,3 @@ func makeFilterConfigByPermission(accessType filterType) func(c *Context, struct
 		return c.App.SessionHasPermissionTo(*c.AppContext.Session(), model.PermissionManageSystem)
 	}
 }
-
-func migrateConfig(c *Context, w http.ResponseWriter, r *http.Request) {
-	props := model.StringInterfaceFromJSON(r.Body)
-	from, ok := props["from"].(string)
-	if !ok {
-		c.SetInvalidParam("from")
-		return
-	}
-	to, ok := props["to"].(string)
-	if !ok {
-		c.SetInvalidParam("to")
-		return
-	}
-
-	auditRec := c.MakeAuditRecord("migrateConfig", audit.Fail)
-	defer c.LogAuditRec(auditRec)
-
-	if !c.App.SessionHasPermissionTo(*c.AppContext.Session(), model.PermissionManageSystem) {
-		c.SetPermissionError(model.PermissionManageSystem)
-		return
-	}
-
-	isCloud := c.App.Srv().License() != nil && *c.App.Srv().License().Features.Cloud
-	if isCloud {
-		c.Err = model.NewAppError("migrateConfig", "api.config.migrate_config_restricted.app_error", nil, "", http.StatusForbidden)
-		return
-	}
-
-	err := config.Migrate(from, to)
-	if err != nil {
-		c.Err = model.NewAppError("migrateConfig", "api.config.migrate_config.app_error", nil, err.Error(), http.StatusInternalServerError)
-		return
-	}
-
-	auditRec.Success()
-	ReturnStatusOK(w)
-}