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 98b3b937 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.
The test also exists in v9.16 branch where rndc.py is available. However, I'm not sure if fixing the test there is worth the effort. I'm inclined to remove it there as well.