arm: imx6: Remove guard around CONFIG_SYS_MMC_ENV_PART

With moving CONFIG_SYS_MMC_ENV_PART to Kconfig we will always have this
be defined with CONFIG_ENV_IS_IN_MMC.  Rather than only do something
when the default is non-zero, we simply drop the guard in this case.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: "NXP i.MX U-Boot Team" <uboot-imx@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>