runner: don't try to teardown previous items from pytest_runtest_setup

While working on improving the documentation of the
`pytest_runtest_setup` hook, I came up with this text:

> Called to perform the setup phase of the test item.
>
> The default implementation runs ``setup()`` on item and all of its
> parents (which haven't been setup yet). This includes obtaining the
> values of fixtures required by the item (which haven't been obtained
> yet).

But upon closer inspection I noticed this line at the start of
`SetupState.prepare` (which is what does the actual work for
`pytest_runtest_setup`):

    self._teardown_towards(needed_collectors)

which implies that the setup phase of one item might trigger teardowns
of *previous* items. This complicates the simple explanation. It also
seems like a completely undesirable thing to do, because it breaks
isolation between tests -- e.g. a failed teardown of one item shouldn't
cause the failure of some other items just because it happens to run
after it.

So the first thing I tried was to remove that line and see if anything
breaks -- nothing did. At least pytest's own test suite runs fine. So
maybe it's just dead code?

src/_pytest/runner.py

@@ -409,16 +409,15 @@ class SetupState:
             raise exc
 
     def prepare(self, colitem) -> None:
-        """ setup objects along the collector chain to the test-method
-            and teardown previously setup objects."""
-        needed_collectors = colitem.listchain()
-        self._teardown_towards(needed_collectors)
+        """Setup objects along the collector chain to the test-method."""
 
         # check if the last collection node has raised an error
         for col in self.stack:
             if hasattr(col, "_prepare_exc"):
                 exc = col._prepare_exc  # type: ignore[attr-defined] # noqa: F821
                 raise exc
+
+        needed_collectors = colitem.listchain()
         for col in needed_collectors[len(self.stack) :]:
             self.stack.append(col)
             try: