Revert "block: Fix unaligned zero write"

This reverts commit fc3959e4.

From upstream commit d01c07f2:
  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>

v2.3.0 does not contain 61007b31 so we can revert the change
directly.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>