powerpc/cell: Remove dead extern declaration for spu_priv1_beat_ops

spu_priv1_beat_ops were removed in commit bf4981a0 ("powerpc: Remove
the celleb support"), remove the unneeded extern.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241112114805.453894-1-mpe@ellerman.id.au