KVM: arm64: Kill off CONFIG_KVM_ARM_HOST

CONFIG_KVM_ARM_HOST is just a proxy for CONFIG_KVM, so remove it in favour
of the latter.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200505154520.194120-2-tabba@google.com