x86/mm: Remove unused NX related declarations

Since commit 4763ed4d ("x86, mm: Clean up and simplify NX enablement")
these declarations is unused and can be removed.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240814031922.2333198-1-yuehaibing@huawei.com

arch/x86/include/asm/pgtable_types.h

@@ -517,8 +517,6 @@ typedef struct page *pgtable_t;
 
 extern pteval_t __supported_pte_mask;
 extern pteval_t __default_kernel_pte_mask;
-extern void set_nx(void);
-extern int nx_enabled;
 
 #define pgprot_writecombine	pgprot_writecombine
 extern pgprot_t pgprot_writecombine(pgprot_t prot);