KVM: Open code kvm_delete_memslot() into its only caller

Fold kvm_delete_memslot() into __kvm_set_memory_region() to free up the
"kvm_delete_memslot()" name for use in a future helper.  The delete logic
isn't so complex/long that it truly needs a helper, and it will be
simplified a wee bit further in upcoming commits.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <2887631c31a82947faa488ab72f55f8c68b7c194.1638817638.git.maciej.szmigiero@oracle.com>