Remove invalid custom ngettext macro

This patch removes all custom gettext macro definitions. There are multiple issues with these macros:

• They are redundant: gettext functions are already declared in <libintl.h>
• The implementations of the ngettext macros are invalid
• If an Xfce GUI app or plugin accidentally uses any of the gettext macros defined in xfce-i18n.h, then it completely disables localization in the app or plugin

The file <libintl.h> containing the declaration of gettext() has been included by both <gi18n.h> and <gi18n-lib.h> since year 2008 at least.

The removal also resolves the following warning messages:

GISCAN Libxfce4util-1.0.gir
xfce-i18n.h:61: Warning: Libxfce4util: symbol='gettext': Unknown namespace for symbol 'gettext'
xfce-i18n.h:62: Warning: Libxfce4util: symbol='dgettext': Unknown namespace for symbol 'dgettext'
xfce-i18n.h:63: Warning: Libxfce4util: symbol='dcgettext': Unknown namespace for symbol 'dcgettext'
xfce-i18n.h:64: Warning: Libxfce4util: symbol='ngettext': Unknown namespace for symbol 'ngettext'
xfce-i18n.h:65: Warning: Libxfce4util: symbol='dngettext': Unknown namespace for symbol 'dngettext'
xfce-i18n.h:66: Warning: Libxfce4util: symbol='dcngettext': Unknown namespace for symbol 'dcngettext'

Closes: #7 (closed)