KVM: arm64: Kill unused vcpu flags field

Horray, we have now sorted all the preexisting flags, and the
'flags' field is now unused. Get rid of it while nobody is
looking.

Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>