[TASK] EM: Remove function menu "Check for extension updates"

The function menu "Check for extension updates" does not make sense anymore
as it is part of the old Extension Manager. Remove it if using the ExtJS-based
Extension Manager.

Change-Id: Ia774e8a3eb8a60ad07c371215ef5d0934a79b7cc
Resolves: #31242
Releases: 4.6
Reviewed-on: http://review.typo3.org/6259
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski
Reviewed-by: Wouter Wolters
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert