base-files: drop support for the platform_nand_pre_upgrade()

No target uses it anymore. All code from that callback was moved into
the platform_do_upgrade().

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

package/base-files/files/lib/upgrade/nand.sh

@@ -282,10 +282,6 @@ nand_upgrade_tar() {
 nand_do_upgrade() {
 	local file_type=$(identify $1)
 
-	if type 'platform_nand_pre_upgrade' >/dev/null 2>/dev/null; then
-		platform_nand_pre_upgrade "$1"
-	fi
-
 	[ ! "$(find_mtd_index "$CI_UBIPART")" ] && CI_UBIPART="rootfs"
 
 	case "$file_type" in