fix placeholder for private blocks

[m1klos]

Fix for #5083

[melnikovi]

Internal ticket created MAGETWO-54528

[adragus-inviqa]

Shouldn't the 2nd arg of sprintf - $output - be removed, too?

Also, since now the actual wrap is not needed, it can have a simpler syntax, something like <!-- BLOCK <name> -->, so that the parser back in page-cache.js can have a more simpler/efficient search-and-replace algorithm.

[okorshenko]

Hi @m1klos. Thank you for your Pull Request. This fix is for deprecated functionality. Please see official documentation Closing this PR.

Thank you @orlangur for linking these issues.