Remove unused systemconf.feature_colours from backend * Remove unused routes for `/system/style/{feature}/{attr}`, including the namespace `systemconf` from the schema. This has previously be replaced by a more general mechanism, which is already used by the frontend.

teichmann@intevation.de = sascha.teichmann@intevation.de
bernhard.reiter@intevation.de = bernhard@intevation.de