KVM: Drop KVM's API to allow temporarily unmapping gfn=>pfn cache

Drop kvm_gpc_unmap() as it has no users and unclear requirements.  The
API was added as part of the original gfn_to_pfn_cache support, but its
sole usage[*] was never merged.  Fold the guts of kvm_gpc_unmap() into
the deactivate path and drop the API.  Omit acquiring refresh_lock as
as concurrent calls to kvm_gpc_deactivate() are not allowed (this is
not enforced, e.g. via lockdep. due to it being called during vCPU
destruction).

If/when temporary unmapping makes a comeback, the desirable behavior is
likely to restrict temporary unmapping to vCPU-exclusive mappings and
require the vcpu->mutex be held to serialize unmap.  Use of the
refresh_lock to protect unmapping was somewhat specuatively added by
commit 93984f19

 ("KVM: Fully serialize gfn=>pfn cache refresh via
mutex") to guard against concurrent unmaps, but the primary use case of
the temporary unmap, nested virtualization[*], doesn't actually need or
want concurrent unmaps.

[*] https://lore.kernel.org/all/20211210163625.2886-7-dwmw2@infradead.org

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>