Ensure that the XDOM doesn't have unnecessary GroupBlocks when parsing

Before we fixed XRENDERING-488, we used to add GroupBlocks inside TableCellBlock, TableHeadCellBlock and ListItemBlock if there were standalone blocks.

However the idea is now to consider that this is a specificity of the XWiki Syntax 2.0+ and thus the XDOM (which should be exactly the same for all parsed syntaxes) should contain those unnecessary Group blocks (they're not necessary in HTML for example).