configs: ethernut5: Drop reiserfs

This is the only board that enables it, and looking generally I don't
believe it's used. All use cases I could fine for the board rub by
default off jffs on the nand and it doesn't enable USB storage.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: egnite GmbH <info@egnite.de>