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@xxxxxxxxxx>