Use upstream gettext instead intltool

With new versions of gettext, there is really no need anymore to use intltool

References:

• https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/166
• https://gitlab.gnome.org/GNOME/gnome-build-meta/issues/104
• https://wiki.gnome.org/Initiatives/GnomeGoals/GettextMigration

Related: !4 (closed)