mm/highmem: Un-EXPORT __kmap_atomic_idx()

Nothing in modules can use that.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095856.595767588@linutronix.de

mm/highmem.c

@@ -108,8 +108,6 @@ static inline wait_queue_head_t *get_pkmap_wait_queue_head(unsigned int color)
 atomic_long_t _totalhigh_pages __read_mostly;
 EXPORT_SYMBOL(_totalhigh_pages);
 
-EXPORT_PER_CPU_SYMBOL(__kmap_atomic_idx);
-
 unsigned int nr_free_highpages (void)
 {
 	struct zone *zone;