Remove system test delzone

There are multiple reasons to remove this test as obsolete:

- The test may not possibly work for over 2.5 years, since
  98b3b93791 removed the rndc.py python
  tool on which this test relies.
- It isn't part of the test suite either in CI or locally unless it is
  explicitly enabled. As a result, there are many issues which prevent
  the test from being executed caused by various refactoring efforts
  accumulated over time.
- Even if the test could be executed, it has no clear failure condition.
  If the python script(s) fail, the test still passes.

bin/tests/system/conf.sh.common

@@ -28,7 +28,7 @@ export LANG=C
 #
 # The "stress" test is not run by default since it creates enough
 # load on the machine to make it unusable to other users.
-# The "dialup", "delzone", and "dupsigs" tests are also not run by
+# The "dialup" and "dupsigs" tests are also not run by
 # default because they take a very long time to complete.
 #