powerpc/configs: Drop unnecessary CONFIG_POWERNV_OP_PANEL

In commit 43a1dd9b ("powerpc/powernv: Add driver for operator
panel on FSP machines") we added CONFIG_POWERNV_OP_PANEL=m to the
powernv defconfig, but it's default m so that's no necessary.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>