KVM: SEV: do not take kvm->lock when destroying

Taking the lock is useless since there are no other references,
and there are already accesses (e.g. to sev->enc_context_owner)
that do not take it.  So get rid of it.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211123005036.2954379-12-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>