Revert "classes/sstate: add a mode to error if sstate package unavailable"

Revert "classes/sstate: add a mode to error if sstate package unavailable"

It turns out that this check cannot work. We don't have the information
to know whether an sstate package is really needed at this point in the
execution, so we check the availability for things that we won't
actually end up needing later on. Thus we can't fail if some of these
aren't found or we'll get needless failures.

This check was intended to give earlier more accurate errors when sstate
artifacts failed to download, but that's not practical so we'll rely
solely on the task execution check that was added within the runqueue.

This reverts most of commit 9e711b54487c3141d7264b8cf0d74f9465020190 (we
still need to allow BB_SETSCENE_ENFORCE through from the external
environment since the eSDK relies upon that.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>