translations: remove duplicate norwegian language

While looking at this website [1], we should be using "nb_NO" and remove this "nb" variant to make it more clear, so we don't need to ship twice same language:

• nb
• nb_NO

Because while trying to add it in WebLate it shows that there is more than one file for Norwegian Bokmål.

[1] http://people.skolelinux.org/pere/blog/Spr_kkoder_for_POSIX_locale_i_Norge.html

Am I right, @comradekingu while looking at https://hosted.weblate.org/languages/nb_NO/ ?