powerpc/32: Drop unused grackle_set_stg()

The call to grackle_set_stg() ("Store Gathering") has always been inside
an #ifdef 0, since the code was first merged in v2.3.43pre7.

Apparently it was suspected of causing problems on some hardware so was
disabled. No one has ever proved otherwise so drop the code as unused
for now.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Closes: https://lore.kernel.org/all/20231031145600.GA9161@bhelgaas/


Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231113051929.1952351-1-mpe@ellerman.id.au