Remove language nb from Makefile

Before this commit, it was creating two same files for Norwegian Bokmål language. When they are present both files, the plugin can not be added to Weblate.

While looking at this website: http://people.skolelinux.org/pere/blog/Spr_kkoder_for_POSIX_locale_i_Norge.html

I see that only nb_NO should be used and nb should not be used, which was confirmed by Norwegian guy.