overlay/031: fix failure with whiteout inode sharing

overlay/031: fix failure with whiteout inode sharing

Kernel commit c21c839b8448 "ovl: whiteout inode sharing" results in
a temp whiteout file resident inside work dir.

Test overlay/031 is a regression test for two user visible bugs:
1. Exposed whiteouts in overlay
2. Failure to remove directory

It also has a sanity tests for a harmless by-product of the bug -
a residue file in work dir.

The new temp whiteout file looks like a residue and causes the test
to fail.

Drop this sanity test, because it is not vital to the regression test.
We could also check if the residue is a single whiteout, but that is
not really needed, so best not poke into overlay internal work dir.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>