Removing the fallback code for best(Session)Diff

[duckaxe]

Related to #1202

The output of bestDiff and bestSessionDiff values includes a check to see if the value is a string, in order to convert it into a number. This legacy code can be removed once NerdAxe provides a numerical value for best(Session)Diff.

[mutatrum]

This legacy code can be removed once NerdAxe provides a numerical value for best(Session)Diff

... and sufficient time has elapsed for people to upgrade.

It could be cleaned up with #1265 and by integrating it into the fallbackDeviceModel (which should be renamed then), so all backwards compatibility code is in a single function.