[BUGFIX] Remove "create new content element" in docheaders

Removes icon in docheader in page backend module to create new
content element. That icon is present in content columns.

Change-Id: I207440ae2a5ba566a1f10203f60a107d74cca3e1
Fixes: #10307
Releases: 4.7, 4.8
Reviewed-on: http://review.typo3.org/9529
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Max Roesch
Tested-by: Max Roesch
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter