Fix config behavior of mng and jasper from "auto" to "no", matching doc

In the configure help, the options for the mng and jasper (jpeg2000)
image handlers are listed as "[no]" - indicating that they are disabled
by default, and must be explicitly enabled. However, the actual
configuration behavior has been that they have been enabled
automatically if the required external libraries were found on the
system - i.e. what the configure help calls "[auto]".

Fix the behavior to match the doc. As these handlers pull in dependency
on external codec libraries that may or may not be well maintained
security-wise, it should be an explicit user decision to enable them.

[ChangeLog][Important Behavior Changes] The mng and jasper (jpeg2000)
image format handlers are no longer built by default if the required
codec libraries are found on the system, but must be explicitly
requested in the configure options. This matches the configure help
documentation for those options.

Task-number: QTBUG-148309
Pick-to: 6.12
Change-Id: I8f4d1cf715bfde11be31aa2de9340986f0a0a2a7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>