shell_integration on OSX: Remove dead code

shell_integration on OSX: Remove dead code

removeIcons isn't called and clearFileNameCacheForPath was always
called with a "nil" path.

Remove the return value of askForIcon which was always 0, and use
that value explicitly at the only call site.

Remove the "-1" code path in iconByPath since setIcons prevents
-1 from getting into _fileNamesCache in all cases.