[BUGFIX] The hgroup element has been removed from HTML5 specifications

css_styled_content is using the hgroup element for grouping the header
and subheader in content elements. Since April 16th, 2013 the hgroup
element has been removed from the HTML5 specifications.

This patch removes the hgroup wrap. There is no alternative currently
but the document outline is still there.

Change-Id: Id9e5116ecdcc9b05ffd6cccfcbbcdcf2599ea5a5
Fixes: #47572
Releases: 6.2
Reviewed-on: https://review.typo3.org/20147
Reviewed-by: Sebastian Fischer
Reviewed-by: Steffen Roßkamp
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn