KVM: arm64: Simplify handling of ARCH_WORKAROUND_2

Owing to the fact that the host kernel is always mitigated, we can
drastically simplify the WA2 handling by keeping the mitigation
state ON when entering the guest. This means the guest is either
unaffected or not mitigated.

This results in a nice simplification of the mitigation space,
and the removal of a lot of code that was never really used anyway.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>