KVM: x86: APICv: drop immediate APICv disablement on current vCPU

Special case of disabling the APICv on the current vCPU right away in
kvm_request_apicv_update doesn't bring much benefit vs raising
KVM_REQ_APICV_UPDATE on it instead, since this request will be processed
on the next entry to the guest.
(the comment about having another #VMEXIT is wrong).

It also hides various assumptions that APIVc enable state matches
the APICv inhibit state, as this special case only makes those states
match on the current vCPU.

Previous patches fixed few such assumptions so now it should be safe
to drop this special case.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210713142023.106183-5-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>