ARM: imx: remove dead left-over from i.MX{27,31,35} removal

The commits:

  commit 879c0e5e ("ARM: imx: Remove i.MX27 board files")
  commit c93197b0 ("ARM: imx: Remove i.MX31 board files")
  commit e1324ece ("ARM: imx: Remove i.MX35 board files")

remove the config MACH_MX27_3DS, MACH_MX31_3DS and MACH_MX35_3DS.
Commit a542fc18 ("ARM: imx31: Remove remaining i.MX31 board code")
further removes arch/arm/mach-imx/3ds_debugboard.{c,h}. So, only some
dead left-over in Kconfig and Makefile remains.

Remove this remaining left-over.

This issue was identified with ./scripts/checkkconfigsymbols.py,
which warns on references to the non-existing configs
MACH_MX{27,31,35}_3DS in ./arch/arm/mach-imx/Kconfig.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>