[BUGFIX] DocumentTemplate class inserts inDocStyles twice

Change-Id: I252da74973c3dc4157717139c95ad0605e16fce1
Releases: 6.2, 6.1, 4.5
Resolves: #55458
Reviewed-on: https://review.typo3.org/27149
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind