Pads are activated automatically when they are added if the element
state is >=PAUSED, so it's not necessary to activate them manually
anymore.
This patch removes manual pad activation from gstaggregator, gstconcat,
gstfunnel, and gstinputselector.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3636>