Don't return the from of a walk

Other storage drivers will only return children and below, s3 should do
the same. The only reason it was returning was because of the addition
of a / to ensure we treat the from as a directory.

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

registry/storage/driver/s3-aws/s3_test.go

@@ -604,7 +604,6 @@ func TestWalk(t *testing.T) {
 			name: "from folder",
 			fn:   func(fileInfo storagedriver.FileInfo) error { return nil },
 			expected: []string{
-				"/folder1",
 				"/folder1/file1",
 			},
 			from: "/folder1",