To delete a file from a solid RAR archive, the archive (at least the files following the deleted one) have to be repacked (compressed again). It is not possible, because the RAR compression algorithm isn’t freely available. You’d need the true WinRAR to do that.

Anyway, this operation doesn’t have much sense for the mentioned installer; if you delete a file from the archive, the application won’t work, because it probably assumes that the keylogger DLL is there.