fixtures: remove unneeded optimization from `_getnextfixturedef`

According to my understanding, this code, which handles obtaining the
relevant fixturedefs when a dynamic `getfixturevalue` is used, has an
optimization where it only grabs fixturedefs that are visible to the
*parent* of the item, instead of the item itself, under the assumption
that a fixturedef can't be visible to a single item, only to a
collector.

Remove this optimization for the following reasons:
- It doesn't save much (one loop iteration in `matchfactories`)
- It slightly complicates the complex fixtures code
- If some plugin wants to make a fixture visible only to a single item,
  why not let it?
- In the static case (`getfixtureclosure`), this optimization is not
  done (despite the confusing name `parentnode`, it is *not* the parent
  node). This is inconsistent.

src/_pytest/fixtures.py

@@ -413,9 +413,7 @@ class FixtureRequest(abc.ABC):
             # We arrive here because of a dynamic call to
             # getfixturevalue(argname) usage which was naturally
             # not known at parsing/collection time.
-            parent = self._pyfuncitem.parent
-            assert parent is not None
-            fixturedefs = self._fixturemanager.getfixturedefs(argname, parent)
+            fixturedefs = self._fixturemanager.getfixturedefs(argname, self._pyfuncitem)
             if fixturedefs is not None:
                 self._arg2fixturedefs[argname] = fixturedefs
         # No fixtures defined with this name.