Remove --cleanuplegacyfadersettings command line option

#2839 created a temporary command line option that performed a clean up of corrupted saved fader settings.  This only needs to be done once per file and users are expected to recover their settings when taking the upgrade from 3.9.0 to 3.9.1.

As part of a later release - after 3.10.0 - this temporary option and the supporting code should be removed.