Remove permanently disabled dds format handler

This handler was disabled in 2016 for security reasons, and has been
unmaintained since. In Qt 6 there are not even build files for it, so
it is effectively dead code. Remove it to avoid false findings from
code analyzers.

Task-number: QTBUG-148309
Pick-to: 6.12
Change-Id: Iaf433c8ee3beb3a28e78d94400a9d18bad8d056e
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>