run-postinsts: do not remove postinsts directory.

When running on the systems having read-only rootfs backed by overlayfs,
removing the whole directory lead to create a special char device file
on the upperdir to reflect directory's removal. Once it is required to
upgrade the whole read-only image that might contain new postinsts scripts,
it will be impossible to run such scripts with a "deletion mark" file
on the overlayfs -- the whole directory will be marked as deleted regardless
new files in it.

Signed-off-by: Anton D. Kachalov <gmouse@google.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>