Avoid unnecessary array operations in update_get_update_list

In update_get_updates_list() and $updates array is created by a call to drupal_get_schema_versions($module) which returns an array of update numbers or FALSE if no updates are pending.

Further in update_get_updates_list() the function array_combine is used to copy the values of this $updates array to the keys and values of the $updates array. Then a foreach loop iterates over the array_keys($updates). This makes the combination of array_combine and array_keys reduntant and means we can just iterate over the $updates array as returned by drupal_get_schema_versions. The case where that variable equals FALSE is handled by a separate if statement guarding this code.