board/ci20/genimage.cfg: drop hardcoded rootfs partition size

As recently reported to the list:
http://lists.busybox.net/pipermail/buildroot/2020-January/271937.html

The hardcoded rootfs partition size can lead to hard to understand build
failures if more packages are added.

So drop the hardcoded partition size.  Genimage will then size the partition
to match the size of the rootfs image (which by default is also 60MB for ext4).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/ci20/genimage.cfg

@@ -24,6 +24,5 @@ image sdcard.img {
         partition-type = 0x83
         image = "rootfs.ext4"
         offset = 2M
-        size = 60M
     }
 }