x86/devmem: Remove duplicate range_is_allowed() definition

17 years ago, Venki suggested [1] "A future improvement would be to
avoid the range_is_allowed duplication".

The only thing preventing a common implementation is that
phys_mem_access_prot_allowed() expects the range check to exit
immediately when PAT is disabled [2]. I.e. there is no cache conflict to
manage in that case. This cleanup was noticed on the path to
considering changing range_is_allowed() policy to blanket deny /dev/mem
for private (confidential computing) memory.

Note, however that phys_mem_access_prot_allowed() has long since stopped
being relevant for managing cache-type validation due to [3], and [4].

Commit 0124cecf ("x86, PAT: disable /dev/mem mmap RAM with PAT") [1]
Commit 9e41bff2 ("x86: fix /dev/mem mmap breakage when PAT is disabled") [2]
Commit 1886297c ("x86/mm/pat: Fix BUG_ON() in mmap_mem() on QEMU/i386") [3]
Commit 0c3c8a18 ("x86, PAT: Remove duplicate memtype reserve in devmem mmap") [4]

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/all/20250430024622.1134277-2-dan.j.williams%40intel.com