x86/mm: Remove duplicated __PAGE_KERNEL(_EXEC) definitions

__PAGE_KERNEL(_EXEC) is defined twice, just remove the superfluous set.

No functional changes.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250714170258.390175-1-nik.borisov@suse.com