wic: Remove mic chroot

wic: Remove mic chroot

mic chroot allows users to chroot into an existing mic image and isn't
used by wic, so remove it.

Removing chroot.py leads in turn to various plugin-loading failures
for a number of plugins that wic doesn't use either, so remove those
as well.

The existing source plugins refer to chroot but don't use it, so fix
those up.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>