cleanup error logging from user_store (#15335)

Tracing this back, it looks like I'm responsible for leaving some spurious logging in the user store.

store/sqlstore/user_store.go

@@ -1134,7 +1134,7 @@ func (us SqlUserStore) GetByUsername(username string) (*model.User, *model.AppEr
 
 	var user *model.User
 	if err := us.GetReplica().SelectOne(&user, queryString, args...); err != nil {
-		return nil, model.NewAppError("SqlUserStore.GetByUsername", "store.sql_user.get_by_username.app_error", nil, err.Error()+" -- "+queryString, http.StatusInternalServerError)
+		return nil, model.NewAppError("SqlUserStore.GetByUsername", "store.sql_user.get_by_username.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}
 
 	return user, nil