Deprecate use of t() for field labels as soon as i18ncck module is committed to i18n project

Deprecate use of t() for field labels as soon as i18ncck module is committed to i18n project.

Before i18ncck, developers wanting to provide translation support for CCK Field labels had to do t($field['widget']['label']). This is misuse of the function t() and it should be removed in favor of i18ncck module that's still pending review in the i18n issues queue.

For further information, please see:
- #531660: i18n + CCK (i18n issue, current status: needs review).
- #531662: Internationalization support (CCK issue, already available since CCK 2.5).
- #499668: Possibility to integrate cck fields? (Translation table issue, already committed to CVS).

For the moment, task postponed until i18ncck module is committed to i18n.