Revert "webui: When building the disk's partition list, also include unmounted partitions in the object's array, otherwise any indexed reference will be wrong since the webui's tree list takes into account all existing partitions, not just mounted ones."

This reverts commit 0843f3ed7243eb00d6852f468ca0dbdd55d01a64.
webui: replace my own hacked fix for non-mountable partition with Asus's own fix backported from 3561, as it's cleaner