Revert "UBUNTU: SAUCE: powerpc/64s: Add support for a store forwarding barrier at kernel entry/exit"

BugLink: https://bugs.launchpad.net/bugs/1830176

This reverts commit 00dcbda8
to be replaced by the upstream implementation.
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>