cmd: nand: Remove not used declaration

This declaration is not used anywhere in the whole tree. There is a
function 'mtd_id_parse()' which was renamed from 'id_parse()' in
commit 68d7d651 ("Separate mtdparts command from jffs2"), but that
function is not used (anymore?) in cmd nand and build is fine without
that declaration, so it's probably just safe to remove.

Signed-off-by: Alexander Dahl <ada@thorsis.com>