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: a35a3c6f ("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>