[BUGFIX] DocumentTemplate class inserts inDocStyles twice

Change-Id: I252da74973c3dc4157717139c95ad0605e16fce1
Releases: 6.2, 6.1, 4.5
Resolves: #55458
Reviewed-on: https://review.typo3.org/27148
Reviewed-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring