[BUGFIX] Hide the field "Selected Pages" for menu type "Sitemap"

The field "Selected Pages" is never used in the content element
"Menu/Sitemap" with menu type "Sitemap". So it should not be
shown in the backend.

Change-Id: I9464ff29fc9fd9864a404cfb14f0545b3e186366
Fixes: #35944
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/10425
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer