MTD users are no longer checking erase_info->state to determine if the
erase operation failed or succeeded. Moreover, mtd_erase_callback() is
now a NOP.

We can safely get rid of all mtd_erase_callback() calls and all
erase_info->state assignments. While at it, get rid of the
erase_info->state field, all MTD_ERASE_XXX definitions and the
mtd_erase_callback() function.
Signed-off-by: NBoris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: NRichard Weinberger <richard@nod.at>
Reviewed-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Acked-by: NBert Kenward <bkenward@solarflare.com>
---
Changes in v2:
- Address a few coding style issues (reported by Miquel)
- Remove comments that are no longer valid (reported by Miquel)