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/rb532/base-files/lib/upgrade/platform.sh

@@ -10,11 +10,8 @@ platform_check_image() {
 	return 0;
 }
 
-platform_nand_pre_upgrade() {
+platform_do_upgrade() {
 	mtd erase kernel
 	tar xf "$1" "sysupgrade-$(board_name)/kernel" -O | nandwrite -o /dev/mtd0 -
-}
-
-platform_do_upgrade() {
 	nand_do_upgrade "$1"
 }