arch/x86: remove CAR global migration when postcar stage is used

When a platform is using postcar stage it's by definition not
tearing down cache-as-ram from within romstage prior to loading
ramstage. Because of this property there's no need to migrate
CAR_GLOBAL variables to cbmem.

Change-Id: I7c683e1937c3397cbbba15f0f5d4be9e624ac27f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19215
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
4 files changed