Fix misrendering of 'nix store --help'

There are no categories underneath 'nix store', so having 'nix store
copy-log' in a category rendered as ':'.

src/nix/store-copy-log.cc

@@ -24,8 +24,6 @@ struct CmdCopyLog : virtual CopyCommand, virtual InstallablesCommand
           ;
     }
 
-    Category category() override { return catUtility; }
-
     void run(ref<Store> srcStore, Installables && installables) override
     {
         auto & srcLogStore = require<LogStore>(*srcStore);