unicode: remove the charset field from struct unicode_map

It is hardcoded and only used for a f2fs sysfs file where it can be
hardcoded just as easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>