Remove DDS from image format docs

The plugin has been removed from the build (since 5.8) and is now
unmaintained. While keeping the code around for adventurous users makes
sense, people who rely on reading the docs should now assume it is gone.

Change-Id: I9df73d899e49cc94fb9316effa80778adbda5e6d
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>