x86/mm: Remove unused __set_memory_prot()

__set_memory_prot() is unused since:

  5c11f00b ("x86: remove memory hotplug support on X86_32")

Let's remove it.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20241212080904.2089632-2-kevin.brodsky@arm.com