Embeds: Don't show embed discovery link on a static front page.

There's currently no iframe content being generated for a static front page. Giving out a link to that isn't an ideal user experience.

Props peterwilsoncc.

Fixes #35194 for trunk.


Built from https://develop.svn.wordpress.org/trunk@36059


git-svn-id: https://core.svn.wordpress.org/trunk@36024 1a063a9b-81f0-0310-95a4-ce76da25c4cd