ARM: imx: cm_fx6: remove esdhc init code from board file

Commit 5248930e ("dm: imx: cm_fx6: Enable more driver model support")
enabled driver model support for MMC. Remove the old mmc init code, which
is no longer used, from the board file.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>