powerpc/powernv: remove get_cxl_module

The static inline get_cxl_module function is entirely unused since commit
8bf6b91a

 ("Revert "powerpc/powernv: Add support for the cxl kernel
api on the real phb"), so remove it.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>