treewide: sysupgrade: get rid of platform_nand_pre_upgrade()

1) nand_do_upgrade() is always called by a target code
2) nand_do_upgrade() starts with calling platform_nand_pre_upgrade()

It means there is no need for the platform_nand_pre_upgrade() callback
at all. All code that was present there could bo moved & simplly called
by a target right before the nand_do_upgrade().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/ramips/base-files/lib/upgrade/ubnt.sh

@@ -73,4 +73,6 @@ platform_upgrade_ubnt_erx() {
 	fi
 
 	ubnt_update_target_kernel ${factory_mtd} ${kernel_part} || exit 1
+
+	nand_do_upgrade "$1"
 }