mtd: nand: Kill the MTD_NAND_IDS Kconfig option

MTD_NAND_IDS is selected by MTD_NAND, which makes it useless. Remove
the Kconfig option and link nand_ids.o into the nand.o object file.
Doing that also prevents creating an extra nand_ids.ko module when
MTD_NAND is activated as a module.

Since nand_ids.c is no longer compiled as a standalone module and the
nand_manuf_ids/nand_flash_ids symbols are only used in nand_base.c, we
can get rid of the MODULE_XXX() and EXPORT_SYMBOL() definitions.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>