[docs] Remove mentions of unique_command_register()
[docs] Remove mentions of unique_command_register()

The unique_command_register() function was replaced by
unique_app_add_command(). The documentation and warnings should
not be mentioning non-existing functions.

GNOME/unique,Distrotech/libunique,Distrotech/libunique,GNOME/unique,GNOME/unique,Distrotech/libunique