MDL-68973 versions: Fix other small details

MDL-68973 versions: Fix other small details

Thanks to Luca Bösch and Matteo Scaramuccia that detected
these 2 minor, but ugly details in core version files:

1) Adding comments pointed to versions (3.x, 20190801...) is not
   recommended, they easily come outdated.
2) Using plugin->release in core is no sense. Core plugins belong
   to the core release.

This commit just gets rid of them. That's all.