KVM: x86: Query vcpu->vcpu_idx directly and drop its accessor, again

Read vcpu->vcpu_idx directly instead of bouncing through the one-line
wrapper, kvm_vcpu_get_idx(), and drop the wrapper.  The wrapper is a
remnant of the original implementation and serves no purpose; remove it
(again) before it gains more users.

kvm_vcpu_get_idx() was removed in the not-too-distant past by commit
4eeef242 ("KVM: x86: Query vcpu->vcpu_idx directly and drop its
accessor"), but was unintentionally re-introduced by commit a54d8066
("KVM: Keep memslots in tree-based structures instead of array-based ones"),
likely due to a rebase goof.  The wrapper then managed to gain users in
KVM's Xen code.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20220614225615.3843835-1-seanjc@google.com