oxnas: drop some unused sysupgrade code

oxnas: drop some unused sysupgrade code

First of all platform_do_upgrade_phase2 doesn't seem to be defined
anywhere. Other than that platform_do_upgrade is not likely to be ever
called. There is a call to nand_do_platform_check which triggers
nand_upgrade_stage1 call with "exit 0" in it.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47038 3c298f89-4303-0410-b956-a3cf2f4a3e73