blk: Drop reference to DM_SPL

The intent here is to only allow SPL_LEGACY_BLK if !SPL_DM - i.e. that
when driver model is enabled in SPL, legacy block cannot be used.

However this combination is used by about 240 boards, so we cannot
disallow it, at least not yet.

So just drop the condition.

Signed-off-by: Simon Glass <sjg@chromium.org>