mmc: core: remove shutdown handler

If mmc/sd shutdown callback suspends the card when card's runtime PM status
is still set to "runtime active" then any request issued after mmc/sd
shutdown will not resume the card hence we will end up issuing the request
to host driver while card isn't in a state to handle it.

Right way to fix this issue is not to allow any requests to flow in after
the shutdown callback has executed. Until it is fixed in the right way, we
are disabling the shutdown handling as it anyways doesn't do real useful
things.

Change-Id: I44681f3c2fd0435bffa393abbbd1e8eacc5a07da
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>