Remove an optimisation to not run the body of ContentPanel::check_selection

if it is called with an unchanged selection.  It breaks updates
of the available tabs when a OV is added to a VF DCP, or when a new CPL
is chosen that has subs where the previous one did not.

Backport from d038abd7a0dce09aa84101c45af5ec8e4086e64f in v2.15.x.