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 93984f19e7 ("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>

include/linux/kvm_host.h

@@ -1333,18 +1333,6 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
  */
 int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
 
-/**
- * kvm_gpc_unmap - temporarily unmap a gfn_to_pfn_cache.
- *
- * @kvm:	   pointer to kvm instance.
- * @gpc:	   struct gfn_to_pfn_cache object.
- *
- * This unmaps the referenced page. The cache is left in the invalid state
- * but at least the mapping from GPA to userspace HVA will remain cached
- * and can be reused on a subsequent refresh.
- */
-void kvm_gpc_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc);
-
 /**
  * kvm_gpc_deactivate - deactivate and unlink a gfn_to_pfn_cache.
  *