Revert "switch_root: add subroot support"

Revert "switch_root: add subroot support"

This reverts commit a692a8745941a192528c5e2a05de97155ba586f9.

On Wed, Nov 18, 2009 at 03:33:12PM +0000, Daniel Drake wrote:
> Booting into a system this way just leads to problems because
> you cannot remount the root read-only at shutdown (leading to unclean
> shutdowns).

> Miklos Szeredi pointed out a trick to turn any directory into a
> mount point which avoids this problem. Therefore we can simplify
> switch_root again and simply document that its users should set