How to remove the php gettext extension

Only when the gettext was installed as a loadable PHP extension. Check your PHP configuration (php.ini) for extension lines. If you want users to be unable to use gettext you can use the PHP disable_functions configuration directive to disable the gettext functions, from the PHP manual:

disable_functions string
This directive allows you to disable certain functions for security reasons. It takes on a comma-delimited list of function names. disable_functions is not affected by Safe Mode. This directive must be set in php.ini For example, you cannot set this in httpd.conf.