sf: Remove unneeded SST_BP and SST_WP

SST parts added on sf_params.c supports both SST_WR which consits
of both BP and WP and there is a spi controller ich which supports
only BP so the relevent _write hook set based on "slave->op_mode_tx"
hence there is no respective change required from flash side hance
removed these.

Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>