`jtdiff` is an effectively obsolete feature that was originally intended to help monitor overall test results in CI systems, by comparing test results across platforms and across builds. It predates OpenJDK, and the environments for which it was intended are long gone.

We should remove the code and the makefile rules to build it.

If it were ever to come back, maybe it could be integrated as a first-class component into jtreg and/or jtreg.jar, instead of being a separate tool and/or jar file.