fat/fs: remove a bunch of dead code

Spotted by chance, when trying to remove file_fat_ls(), I noticed there
were some dead users of the API.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Simon Glass <sjg@chromium.org>