Doc: remove list of available Qt plugins

The list is outdated. Will move this information gradually
to module docs and add links to the table that lists the
base plugin classes and is updated in
924d4cd7ca5c5d226f0f195990297509353515ea.

Change-Id: I40962504ac4399ec69cd48b0ca5c74ad987b92dc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>