ARM: imx_v6_v7_defconfig: Remove explicit ARM_UNWIND disable

CONFIG_ARM_UNWIND is removed when running 'savedefconfig', but
selected by ARM EABI (AEBI) support.  This is done in preparation to making
further changes to this defconfig cleaner.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>