nand_spl: remove unused linker scripts u-boot-nand.lds

Commit 345b77ba removed some nand_spl boards but
it missed to delete linker scripts.

These linker scripts are not used now.

And one more fix:
amcc/acadia does not support nand_spl anymore, so remove
  #if !defined(CONFIG_NAND_U_BOOT) ... #endif

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>