mtd: gpmi: delete the gpmi_pre_bbt_scan

We do not scan the BBT after we call the gpmi_pre_bbt_scan,
so it has lost the meaning of existence.

This patch merges this function into gpmi_init_last, and delete it.
This patch does not change any logic.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>