encoding: add private function remove_duplicates()

It'll be used later.
Ensure that UTF-8 and CURRENT are present in the candidate encodings

In the gedit translations, CURRENT was sometimes translated (and there
is a translator comment to explain that it should not be translated...).
So it's better to ensure that UTF-8 and CURRENT are present, just in
case.
FileLoader: remove duplicated candidate encodings

It'll be useful in gedit to prepend the metadata and GtkSourceFile's
encodings, without the need to make
_gtk_source_encoding_remove_duplicates() public.
Make gtk_source_encoding_get_default_candidates() private

It is currently not used in gedit. Let's wait the next development
cycle to see if it is really needed to make the function public.