Fix disabling of groups (#709)

This was broken with commit 5897285, which reverted the changes in
commit c6dacb1, but rather than only removing the change concerning
the read-only attribute, commented out the entire check, including
the parent_->getDisableChildren() call (which existed prior to
commit 5897285).
Merge pull request #769 from ros-visualization/hydro_backport_740

Hydro backport of #740