Remove php-gettext code

[westi]

We now use pomo to parse the translation files so we no longer need the php-gettest library

We should remove it and announce it's not there.

We need to remove wp-includes/gettext.php and wp-includes/streams.php

[westi]

(In [12006]) Remove the php-gettext library as we don't use it anymore.

[westi]

(In [12604]) Remove the gettext libs that accidentally got added back in. See #10890.

[hakre]

Those were re-introduced in [12603] by wpmuguru just today. #11644