powerpc: remove the __kernel_io_end export

This export was added in this merge window, but without any actual
user, or justification for a modular user.

Fixes: a35a3c6 ("powerpc/mm/hash64: Add a variable to track the end of IO mapping")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Author: Christoph Hellwig

arch/powerpc/mm/pgtable_64.c

@@ -97,7 +97,6 @@ EXPORT_SYMBOL(__vmalloc_end);
 unsigned long __kernel_io_start;
 EXPORT_SYMBOL(__kernel_io_start);
 unsigned long __kernel_io_end;
-EXPORT_SYMBOL(__kernel_io_end);
 struct page *vmemmap;
 EXPORT_SYMBOL(vmemmap);
 unsigned long __pte_frag_nr;