mvebu: bubt: Drop dead code

The code around CONFIG_SYS_MMC_ENV_PART has been untested since merge.
This can be seen by it referencing 'mmc->part_num' which was migrated
elsewhere prior to this code being merged.

Cc: Joel Johnson <mrjoel@lixil.net>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>