bcm53xx: drop dead upgrade code

platform_nand_pre_upgrade() is gone since commit 790692dde2
("base-files: drop support for the platform_nand_pre_upgrade()").

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit a6dc0f680d)

target/linux/bcm53xx/base-files/lib/upgrade/platform.sh

@@ -403,11 +403,3 @@ platform_do_upgrade() {
 		;;
 	esac
 }
-
-platform_nand_pre_upgrade() {
-	case "$(board_name)" in
-	meraki,mr32)
-		CI_KERNPART="part.safe"
-		;;
-	esac
-}