IBM Akebono: Remove obsolete config select

The original implementation of MMC support for Akebono introduced a
new configuration symbol (MMC_SDHCI_OF_476GTR). This symbol has been
dropped in favour of using the generic platform driver however the
select for this symbol was mistakenly left in the platform
configuration.

This patch removes the obsolete symbol selection.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>