Use global diff tag variables instead of KBABEL strings, delete dead code.

Diffs are represented with curly brace tags but the tags are typically handwritten rather than using the preexisting constant variables in diff.cpp. This changes the tags to use the variable where possible, and also deletes some code that has been commented out for 15+ years. There should be no change in functionality for the end user, these changes should just make code maintenance and contributions easier.