[kcms/kdrb] Drop legacy colour sync in rdb

The intention of this code was to sync with a selection of really really old applications.

Colours wuold be written into xrdb but there was also a hook to store and load defaults for a specific set of really legacy applications, initally defined in '98 and not updated much since.

This has become very outdated, such as tell xpdf to use kfmclient which no longer exists.