mx31: remove mx31moboard_defconfig

We want to have a mx31_defconfig file that builds a kernel that is able
to boot on all support mx31 systems and thus also can be better tested
by automatic build scripts. For these reasons, this config file is not
needed anymore.

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>