ProjectExplorerPlugin:addProject() remove code to set startup project

addProject() is called from 2 places: openProject and restoreSession
openProject() takes care of setting a startup project itself and
restoreSession sets it explicitly after opening the projects. So the
code in addProject() is not needed.

Change-Id: Iaa5de3e36fd4b60c8ceb79b8ea3ca68dbc9b491c
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>