[settings] Fix CSettings::UninitializeISettingsHandlers

[ksooo]

CSettings::UninitializeISettingsHandlers was a complete mess (maybe copy/paste error initially and than things went mad).

The method now:

• deregisters settings handlers, not settings callbacks (c/p error in initial implementation?)
• deregisters the handlers actually registered in CSettings::InitializeISettingsHandlers
• uses exactly the revers order of registration to deregister

I runtime-tested the changes on macOS, latest Kodi master.

@Montellese if you are araound, pleaes take a look. If not, I will merge in a couple of days.

[garbear]

@Montellese confirmed this was an implementation error when I noticed it a while ago, but I forgot to finish correcting this. Change looks good.

[ksooo]

@garbear thanks for taking a look.