qcow2: Remove wrong metadata overlap check

In qcow2_write_compressed, if the compression fails, a normal cluster is
written to disk. This is done through bdrv_write on the qcow2 BDS
itself (using the guest offset), thus it is wrong to do a metadata
overlap check before.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
