mtd: Stop updating erase_info->state and calling mtd_erase_callback()

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: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Bert Kenward <bkenward@solarflare.com>
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Changes in v2:
- Address a few coding style issues (reported by Miquel)
- Remove comments that are no longer valid (reported by Miquel)