Remove prereq.sh support from the system test runner

With every prereq.sh converted to pytest markers, the conftest
fixture no longer needs to locate and run a per-directory prereq.sh.
Drop check_prerequisites() and the README entry for the file.

Assisted-by: Claude:claude-opus-4-8
(cherry picked from commit 1cc95fe091)

bin/tests/system/README.md

@@ -229,9 +229,6 @@ ported to python; do not add them to new tests:
 - `setup.sh`: Legacy shell test setup.  New tests use templates and a
   `bootstrap()` function instead.
 
-- `prereq.sh`: Legacy prerequisite check; when it exits non-zero, the test is
-  skipped.  New tests use pytest marks (see `isctest/mark.py`).
-
 ### Module Scope
 
 A module is a python file which contains test functions. Every system

bin/tests/system/conftest.py

@@ -546,14 +546,6 @@ def system_test(
             isctest.log.error("testsock.pl: exited with code %d", exc.returncode)
             pytest.skip("Network interface aliases not set up.")
 
-    def check_prerequisites():
-        try:
-            isctest.run.shell(f"{system_test_dir}/prereq.sh")
-        except FileNotFoundError:
-            pass  # prereq.sh is optional
-        except subprocess.CalledProcessError:
-            pytest.skip("Prerequisites missing.")
-
     def setup_test():
         template_data = None
         bootstrap_fn = getattr(request.module, "bootstrap", None)
@@ -613,7 +605,6 @@ def system_test(
 
     # Perform checks which may skip this test.
     check_net_interfaces()
-    check_prerequisites()
 
     # Store the fact that this fixture hasn't successfully finished yet.
     # This is checked before temporary directory teardown to decide whether