Temporarily revert the SELinux policy for persist.netd.stable_secret.

This change did not make it into core sepolicy in time for O.
The revert allows devices to define these selinux policies in
vendor-specific sepolicy instead of core sepolicy. It is
necessary because:

1. It is too late to change property_contexts in O.
2. Adding the netd_stable_secret prop to vendor sepolicy results
   in a duplicate definition error at compile time.
3. Defining a new vendor-specific context (such as
   net_stable_secret_vendor_prop) and applying it to
   persist.netd.stable_secret results in the device not booting
   due to attempting to apply two different contexts to the same
   property.

Lack of the sepolicy no longer breaks wifi connectivity now that
IpManager no longer considers failure to set the stable secret to
be a fatal error.

Once all interested devices have adopted the vendor sepolicy,
this policy can safely be reinstated by reverting said vendor
sepolicies in internal master.

This reverts commit abb1ba65.

Bug: 17613910
Test: bullhead builds, boots, connects to wifi
Change-Id: Idffcf78491171c54bca9f93cb920eab9b1c47709