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.
Built from https://develop.svn.wordpress.org/trunk@30285


git-svn-id: http://core.svn.wordpress.org/trunk@30285 1a063a9b-81f0-0310-95a4-ce76da25c4cd