Revert "block: Fix unaligned zero write"

This reverts commit fc3959e4.

The core write code already handles the case, so remove this
duplication.

Because commit 61007b31 moved the touched code from block.c to
block/io.c, the change is manually reverted.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1431522721-3266-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>