[REF] web: remove legacy rejectAfter

In our continued effort to remove legacy code, this commit remove the
the legacy rejectAfter util as it's not used anymore.

Part-of task-id 3439226

Part-of: odoo/odoo#132559

addons/web/static/src/legacy/js/core/concurrency.js

@@ -152,20 +152,6 @@ const concurrency = {
             }
         }
     }),
-    /**
-     * Rejects a promise as soon as a reference promise is either resolved or
-     * rejected
-     *
-     * @param {Promise} [target_def] the promise to potentially reject
-     * @param {Promise} [reference_def] the reference target
-     * @returns {Promise}
-     */
-    rejectAfter: function (target_def, reference_def) {
-        return new Promise(function (resolve, reject) {
-            target_def.then(resolve).guardedCatch(reject);
-            reference_def.then(reject).guardedCatch(reject);
-        });
-    }
 };
 
 export default concurrency;