cmd_mmc.c: Drop open/close mmc sub-commands

The open and close mmc sub-commands implement a hard-coded set of values
specific to the SMDK5250 platform.  Remove these commands as what they
did can be done instead with a series of mmc dev / bootpart / bootbus
commands instead now.

Cc: Amar <amarendra.xt@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>