Remove failing test related to post galleries.

It relied on the old _WPDataset technique, which is no longer used.

The test has been added to #UT30 as a patch, in case anyone wants to make
future use of it.

See #30284.

git-svn-id: https://develop.svn.wordpress.org/trunk@30285 602fd350-edb4-49c9-b593-d223f7449a82