please revert merging of PLIP 20256 (social media meta tags)

As already discussed in https://dev.plone.org/ticket/20256, merging of 
this feature into the Plone core sets a dangerous precedent, IMO.

If we feel Dublin Core is no longer relevant, it has to be replaced by a 
new open standard like Linked Open Data and not by social media meta 
tags that are not open, but defined by a handful of big corporations 
that are trying to control the internet.

Including things like Twitter Cards in the core is going to be 
dangerous, painful and needless; we have add-ons for that are more 
flexible and easier to maintain.

Please revert this and let's continue discussing options.

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