UPSTREAM: arm64: KVM: Move away from the assembly version of the world switch

This is it. We remove all of the code that has now been rewritten.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
(cherry picked from commit 1ea66d27)
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>