Remove handling of collection errors by --sw

Since then pytest itself adopted the behavior of interrupting
the test session on collection errors, so --sw no longer needs
to handle this.

The --sw behavior seems have been implemented when pytest
would continue execution even if there were collection errors.

src/_pytest/stepwise.py

@@ -71,12 +71,6 @@ class StepwisePlugin:
 
         config.hook.pytest_deselected(items=already_passed)
 
-    def pytest_collectreport(self, report):
-        if self.active and report.failed:
-            self.session.shouldstop = (
-                "Error when collecting test, stopping test execution."
-            )
-
     def pytest_runtest_logreport(self, report):
         # Skip this hook if plugin is not active or the test is xfailed.
         if not self.active or "xfail" in report.keywords: