I found it simpler to just delete files in the skins directory. I was warned on about 5 "old" skins after upgrading. I just went through the list of warnings and deleted the files. That seemed to be sufficient to keep my wiki running. I deleted both the main file in /skins directory as well as all support files and I ceased to get error messages.
Topic on Manual talk:Skin autodiscovery
Upgrading to 1.23
I don't think this is a very good idea, especially for inexperienced users since 1.23 still looks for the skins in /skins if nothing else is specified in LocalSettings.php. and of one of the affected skins is used, especially as default skin. On my local installation deletion (or renaming) of the skins folder and or the Vector.php etc files caused a fatal error and Skin not found warnings.
Not sure why it worked for you, though. I guess you are using a custom skin or one that is already compatible?
The bundled skins will work and it's not that hard to migrate customs skins.
Thanks for the input. I'd guess I don't have any users who specify their skin so the skins I deleted were never requested. I have no custom skins. HTH.
First: As far as old core skins are concerned, Upgrade already tells us to remove these according files and folders as far as these skins have been removed from MediaWiki itself. These files belong to the category "old files left behind, which may cause trouble".
Second: As far as in contrast custom skins are concerned, these should be upgraded as described at Manual:Skin_autodiscovery. You surely can remove them as you described above, but doing so basically just uninstalls the skin. In your case that obviously was OK - for others it may be no option.