iotests: Fix duplicated diff output on failure

Commit 70ff5b07 wanted to move the diff between actual and reference
output to the end after printing the test result line. It really only
copied it, though, so the diff is now displayed twice. Remove the old
one.

Fixes: 70ff5b07fcdd378180ad2d5cc0b0d5e67e7ef325
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/check

@@ -876,7 +876,6 @@ do
                     fi
                 else
                     mv $tmp.out $seq.out.bad
-                    $diff -w "$reference" "$PWD"/$seq.out.bad
                     status="fail"
                     results="output mismatch (see $seq.out.bad)"
                     printdiff=true