mips: delete duplicated BUILTIN_DTB and LIBFDT configs

CONFIG_BUILTIN_DTB and CONFIG_LIBFDT selection is duplicated
in menu "Machine selection" under MIPS_MALTA.

Signed-off-by: Maksym Kokhan <maksym.kokhan@globallogic.com>
Signed-off-by: Andrii Bordunov <andrew.bordunov@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21100/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org