Revert "Restore About dialog for plugins in "Loaded Plugins" dialog"

This code needs a newer version of KCoreAddons, so we can't do it
in the 5.0 branch.

This reverts commits 9fe2db57aede3cf79ee521f4f9e167dfed6306bd
and e565620aae24ccdb4e2a1641865e1633731c9740.