mtd: rawnand: remove STANDALONE compile mode of nand_ecc

This file can not be built independently any more.  We would have to
bring in more to resolve mtd_to_nand(mtd)->ecc.size, for example.

It is difficult to notice a breakage since nobody compiles this mode.
It is not worth fixing or maintaining in my opinion.  Remove.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>