feat(userMgr): add user removal functionality

- Implement RemoveUser function in database.go to delete users
- Add RemoveUserHandler to handle HTTP requests for user removal
- Update API documentation with new /userMgr/remove endpoint
- Add route for user removal in router.go

router.go

@@ -13,6 +13,7 @@ func setupRoutes() {
 	http.HandleFunc("/logout", LogoutHandler)
 
 	http.HandleFunc("/userMgr/create", CreateUserHandler)
+	http.HandleFunc("/userMgr/remove", RemoveUserHandler)
 
 	http.HandleFunc("/frpcAct/instanceMgr/create", CreateInstanceHandler)
 	http.HandleFunc("/frpcAct/instanceMgr/list", ListInstancesHandler)