mx51evk: Do not force the rootfs type

Currently mmcrootfstype is set to ext3 type.

It is better not to force it in the env vars, because users may prefer a
different file system type, so let's get rid of 'mmcrootfstype'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>