Pings/Trackbacks: Remove a failing test that won't pass as long as...

Pings/Trackbacks: Remove a failing test that won't pass as long as `WP_IMPORTING` gets set during tests.

The `_publish_post_hook()` function checks for `WP_IMPORTING` before setting meta fields fir enclosures and pings, which means this test is doomed to fail.

The test can be re-implemented if the `WP_IMPORTING` constant gets moved to a function similar to how `wp_installing()` works for the `WP_INSTALLING` constant.

See #48014

Built from https://develop.svn.wordpress.org/trunk@46428


git-svn-id: https://core.svn.wordpress.org/trunk@46226 1a063a9b-81f0-0310-95a4-ce76da25c4cd