mtd: rawnand: remove ->legacy.erase and single_erase()

Now that the last user of this hook, denali.c, stopped using it,
we can remove the erase hook from nand_legacy.

I squashed single_erase() because only the difference between
single_erase() and nand_erase_op() is the number of bit shifts.

The status/ret conversion in nand_erase_nand() is unneeded since
commit eb94555e

 ("mtd: nand: use usual return values for the
->erase() hook"). Cleaned it up now.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>