sysroot: Don't cache sepolicy

sysroot: Don't cache sepolicy

In [this commit](https://github.com/ostreedev/ostree/commit/6ce80f9685e6273dbcb4731d6650a10976ea220a)
for some reason I added a `sepolicy` member to the sysroot.  I
have no idea why I did that, and it's conceptually wrong
since the policy is specific to a *deployment*.

This bit me when I was working on [a pull request](https://github.com/ostreedev/ostree/pull/763)
elsewhere, since at that point it was `NULL`.

We already pass around the sepolicy in the deployment code, so just stop caching
it.

Closes: #764
Approved by: jlebon