base-files: get rid of forgotten traces of fitblk_get_bootdev

The function fitblk_get_bootdev doesn't exist any more, using it in
export_bootdevice anyway never made much sense and only worked for
classic block devices.
Just drop /dev/fit* handling there, it isn't needed anywhere.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

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

@@ -196,7 +196,6 @@ export_bootdevice() {
 			done
 		;;
 		/dev/*)
-			[ "$rootpart" = "/dev/fit0" ] && rootpart="$(fitblk_get_bootdev)"
 			uevent="/sys/class/block/${rootpart##*/}/../uevent"
 		;;
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