Block Editor: Remove experimental Social Links blocks

We decided to mark Social Links block as an experimental feature in Gutenberg. In effect, we are moving all related code from WordPress 5.3 release.

Props youknowriad.

Fixes #48263.


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git-svn-id: https://core.svn.wordpress.org/trunk@46340 1a063a9b-81f0-0310-95a4-ce76da25c4cd